diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index cd9c64594..02a9ea701 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -16,9 +16,9 @@ namespace nix {
     
 
 EvalState::EvalState()
-    : normalForms(32768), primOps(128)
+    : sessionNormalForms(32768), normalForms(32768), primOps(128)
 {
-    nrEvaluated = nrCached = 0;
+    nrEvaluated = nrCached = nrDephtAfterReset = 0;
 
     initNixExprHelpers();
 
@@ -803,10 +803,17 @@ Expr evalExpr(EvalState & state, Expr e)
 #endif
 
     state.nrEvaluated++;
+    state.nrDephtAfterReset++;
 
     /* Consult the memo table to quickly get the normal form of
        previously evaluated expressions. */
     Expr nf = state.normalForms.get(e);
+    if (nf) {
+        state.nrCached++;
+        return nf;
+    }
+
+    nf = state.sessionNormalForms.get(e);
     if (nf) {
         if (nf == makeBlackHole())
             throwEvalError("infinite recursion encountered");
@@ -815,14 +822,22 @@ Expr evalExpr(EvalState & state, Expr e)
     }
 
     /* Otherwise, evaluate and memoize. */
-    state.normalForms.set(e, makeBlackHole());
+    state.sessionNormalForms.set(e, makeBlackHole());
     try {
         nf = evalExpr2(state, e);
     } catch (Error & err) {
-        state.normalForms.remove(e);
+        state.sessionNormalForms.remove(e);
         throw;
     }
-    state.normalForms.set(e, nf);
+
+    if (state.nrDephtAfterReset) {
+        state.sessionNormalForms.remove(e);
+        state.normalForms.set(e, nf);
+        state.nrDephtAfterReset--;
+    } else {
+        state.sessionNormalForms.set(e, nf);
+    }
+
     return nf;
 }
 
@@ -830,6 +845,7 @@ Expr evalExpr(EvalState & state, Expr e)
 Expr evalFile(EvalState & state, const Path & path)
 {
     startNest(nest, lvlTalkative, format("evaluating file `%1%'") % path);
+    state.nrDephtAfterReset = 0;
     Expr e = parseExprFromFile(state, path);
     try {
         return evalExpr(state, e);
@@ -901,11 +917,13 @@ void printEvalStats(EvalState & state)
     char x;
     bool showStats = getEnv("NIX_SHOW_STATS", "0") != "0";
     printMsg(showStats ? lvlInfo : lvlDebug,
-        format("evaluated %1% expressions, %2% cache hits, %3%%% efficiency, used %4% ATerm bytes, used %5% bytes of stack space")
+        format("evaluated %1% expressions, %2% cache hits, %3%%% efficiency, used %4% ATerm bytes, used %5% bytes of stack space, %6% normal reduction, %7% session dependent reduction.")
         % state.nrEvaluated % state.nrCached
         % ((float) state.nrCached / (float) state.nrEvaluated * 100)
         % AT_calcAllocatedSize()
-        % (&x - deepestStack));
+        % (&x - deepestStack)
+        % state.normalForms.size()
+        % state.sessionNormalForms.size());
     if (showStats)
         printATermMapStats();
 }
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index fed6d3472..c8557a4b3 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -29,6 +29,7 @@ typedef Expr (* PrimOp) (EvalState &, const ATermVector & args);
 
 struct EvalState 
 {
+    ATermMap sessionNormalForms;
     ATermMap normalForms;
     ATermMap primOps;
     DrvRoots drvRoots;
@@ -37,6 +38,7 @@ struct EvalState
 
     unsigned int nrEvaluated;
     unsigned int nrCached;
+    unsigned int nrDephtAfterReset;
 
     bool allowUnsafeEquality;
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 016e6abdb..223954f72 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -74,12 +74,14 @@ static Expr prim_null(EvalState & state, const ATermVector & args)
    platforms. */
 static Expr prim_currentSystem(EvalState & state, const ATermVector & args)
 {
+    state.nrDephtAfterReset = 0;
     return makeStr(thisSystem);
 }
 
 
 static Expr prim_currentTime(EvalState & state, const ATermVector & args)
 {
+    state.nrDephtAfterReset = 0;
     return ATmake("Int(<int>)", time(0));
 }
 
@@ -228,6 +230,7 @@ static Expr prim_addErrorContext(EvalState & state, const ATermVector & args)
 static Expr prim_getEnv(EvalState & state, const ATermVector & args)
 {
     string name = evalStringNoCtx(state, args[0]);
+    state.nrDephtAfterReset = 0;
     return makeStr(getEnv(name));
 }
 
@@ -316,6 +319,7 @@ static Hash hashDerivationModulo(EvalState & state, Derivation drv)
 static Expr prim_derivationStrict(EvalState & state, const ATermVector & args)
 {
     startNest(nest, lvlVomit, "evaluating derivation");
+    state.nrDephtAfterReset = 0;
 
     ATermMap attrs;
     queryAllAttrs(evalExpr(state, args[0]), attrs, true);
@@ -549,6 +553,7 @@ static Expr prim_storePath(EvalState & state, const ATermVector & args)
 {
     PathSet context;
     Path path = canonPath(coerceToPath(state, args[0], context));
+    state.nrDephtAfterReset = 0;
     if (!isInStore(path))
         throw EvalError(format("path `%1%' is not in the Nix store") % path);
     Path path2 = toStorePath(path);
@@ -565,6 +570,7 @@ static Expr prim_pathExists(EvalState & state, const ATermVector & args)
     Path path = coerceToPath(state, args[0], context);
     if (!context.empty())
         throw EvalError(format("string `%1%' cannot refer to other paths") % path);
+    state.nrDephtAfterReset = 0;
     return makeBool(pathExists(path));
 }
 
@@ -598,6 +604,7 @@ static Expr prim_readFile(EvalState & state, const ATermVector & args)
     Path path = coerceToPath(state, args[0], context);
     if (!context.empty())
         throw EvalError(format("string `%1%' cannot refer to other paths") % path);
+    state.nrDephtAfterReset = 0;
     return makeStr(readFile(path));
 }
 
@@ -637,6 +644,7 @@ static Expr prim_toFile(EvalState & state, const ATermVector & args)
         refs.insert(path);
     }
     
+    state.nrDephtAfterReset = 0;
     Path storePath = readOnlyMode
         ? computeStorePathForText(name, contents, refs)
         : store->addTextToStore(name, contents, refs);
@@ -689,6 +697,7 @@ static Expr prim_filterSource(EvalState & state, const ATermVector & args)
 
     FilterFromExpr filter(state, args[0]);
 
+    state.nrDephtAfterReset = 0;
     Path dstPath = readOnlyMode
         ? computeStorePathForPath(path, true, htSHA256, filter).first
         : store->addToStore(path, true, htSHA256, filter);