diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index 8606da5597..e8e4c050cc 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -46,7 +46,7 @@ Expr findAlongAttrPath(EvalState & state, const string & attrPath,
 
         if (apType == apAttr) {
 
-            ATermMap attrs(128);
+            ATermMap attrs;
 
             if (!isAttrs(state, e, attrs))
                 throw TypeError(
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index e58ff695c5..8665edcfab 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -37,7 +37,7 @@ static Expr substArgs(EvalState & state,
     ATermMap subs(nrFormals);
 
     /* Get the actual arguments and put them in the substitution. */
-    ATermMap args(128); /* !!! fix */
+    ATermMap args;
     queryAllAttrs(arg, args);
     for (ATermMap::const_iterator i = args.begin(); i != args.end(); ++i)
         subs.set(i->key, i->value);
@@ -151,7 +151,7 @@ static Expr updateAttrs(Expr e1, Expr e2)
 {
     /* Note: e1 and e2 should be in normal form. */
 
-    ATermMap attrs(128); /* !!! */
+    ATermMap attrs;
     queryAllAttrs(e1, attrs, true);
     queryAllAttrs(e2, attrs, true);
 
@@ -343,7 +343,7 @@ Expr autoCallFunction(Expr e, const ATermMap & args)
     ATerm body, pos;
     
     if (matchFunction(e, formals, body, pos)) {
-        ATermMap actualArgs(128);
+        ATermMap actualArgs(ATgetLength(formals));
         
         for (ATermIterator i(formals); i; ++i) {
             Expr name, def, value; ATerm values, def2;
@@ -496,7 +496,7 @@ Expr evalExpr2(EvalState & state, Expr e)
 
     /* Withs. */
     if (matchWith(e, e1, e2, pos)) {
-        ATermMap attrs(128); /* !!! */
+        ATermMap attrs;        
         try {
             e1 = evalExpr(state, e1);
             queryAllAttrs(e1, attrs);
@@ -550,7 +550,7 @@ Expr evalExpr2(EvalState & state, Expr e)
 
     /* Attribute existence test (?). */
     if (matchOpHasAttr(e, e1, name)) {
-        ATermMap attrs(128); /* !!! */
+        ATermMap attrs;
         queryAllAttrs(evalExpr(state, e1), attrs);
         return makeBool(attrs.get(name) != 0);
     }
@@ -576,7 +576,7 @@ Expr evalExpr2(EvalState & state, Expr e)
             if (matchAttrs(e1, as) && matchPath(e2, p)) {
                 static bool haveWarned = false;
                 warnOnce(haveWarned, format(
-                    "concatenation of a derivation and a path is deprecated, "
+                    "concatenation of a derivation and a path is deprecated; "
                     "you should write `drv + \"%1%\"' instead of `drv + %1%'")
                     % aterm2String(p));
                 PathSet context;
diff --git a/src/libexpr/expr-to-xml.cc b/src/libexpr/expr-to-xml.cc
index f5b0e74770..5c5b81b238 100644
--- a/src/libexpr/expr-to-xml.cc
+++ b/src/libexpr/expr-to-xml.cc
@@ -44,7 +44,7 @@ static void printTermAsXML(Expr e, XMLWriter & doc, PathSet & context)
 
     else if (matchAttrs(e, as)) {
         XMLOpenElement _(doc, "attrs");
-        ATermMap attrs(128);
+        ATermMap attrs;
         queryAllAttrs(e, attrs);
         StringSet names;
         for (ATermMap::const_iterator i = attrs.begin(); i != attrs.end(); ++i)
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index 3c7061fcd3..ba9fbcd6df 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -43,7 +43,7 @@ MetaInfo DrvInfo::queryMetaInfo(EvalState & state) const
     Expr a = attrs->get(toATerm("meta"));
     if (!a) return meta; /* fine, empty meta information */
 
-    ATermMap attrs2(16); /* !!! */
+    ATermMap attrs2;
     queryAllAttrs(evalExpr(state, a), attrs2);
 
     for (ATermMap::const_iterator i = attrs2.begin(); i != attrs2.end(); ++i) {
@@ -81,7 +81,7 @@ static bool getDerivation(EvalState & state, Expr e,
         e = evalExpr(state, e);
         if (!matchAttrs(e, es)) return true;
 
-        boost::shared_ptr<ATermMap> attrs(new ATermMap(32)); /* !!! */
+        boost::shared_ptr<ATermMap> attrs(new ATermMap());
         queryAllAttrs(e, *attrs, false);
     
         Expr a = attrs->get(toATerm("type"));
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 17fc6b5231..bc6e290a48 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -21,7 +21,7 @@ static Expr primBuiltins(EvalState & state, const ATermVector & args)
        calling a primop `foo' directly, they could say `if builtins ?
        foo then builtins.foo ... else ...'. */
 
-    ATermMap builtins(128);
+    ATermMap builtins(state.primOps.size());
 
     for (ATermMap::const_iterator i = state.primOps.begin();
          i != state.primOps.end(); ++i)
@@ -133,7 +133,7 @@ static Expr primDerivationStrict(EvalState & state, const ATermVector & args)
 {
     startNest(nest, lvlVomit, "evaluating derivation");
 
-    ATermMap attrs(128); /* !!! */
+    ATermMap attrs;
     queryAllAttrs(evalExpr(state, args[0]), attrs, true);
 
     /* Figure out the name already (for stack backtraces). */
@@ -303,7 +303,7 @@ static Expr primDerivationStrict(EvalState & state, const ATermVector & args)
 static Expr primDerivationLazy(EvalState & state, const ATermVector & args)
 {
     Expr eAttrs = evalExpr(state, args[0]);
-    ATermMap attrs(128); /* !!! */
+    ATermMap attrs;    
     queryAllAttrs(eAttrs, attrs, true);
 
     attrs.set(toATerm("type"),
@@ -625,7 +625,7 @@ static Expr primGetEnv(EvalState & state, const ATermVector & args)
    list of strings. */
 static Expr primAttrNames(EvalState & state, const ATermVector & args)
 {
-    ATermMap attrs(128); /* !!! */
+    ATermMap attrs;
     queryAllAttrs(evalExpr(state, args[0]), attrs);
 
     StringSet names;
@@ -689,7 +689,7 @@ static Expr primHasAttr(EvalState & state, const ATermVector & args)
 
 static Expr primRemoveAttrs(EvalState & state, const ATermVector & args)
 {
-    ATermMap attrs(128); /* !!! */
+    ATermMap attrs;
     queryAllAttrs(evalExpr(state, args[0]), attrs, true);
     
     ATermList list = evalList(state, args[1]);
diff --git a/src/libutil/aterm-map.hh b/src/libutil/aterm-map.hh
index 3035b6ec55..db36da377a 100644
--- a/src/libutil/aterm-map.hh
+++ b/src/libutil/aterm-map.hh
@@ -40,7 +40,7 @@ public:
 
     /* Create a map.  `expectedCount' is the number of elements the
        map is expected to hold. */
-    ATermMap(unsigned int expectedCount);
+    ATermMap(unsigned int expectedCount = 16);
     
     ATermMap(const ATermMap & map);