diff --git a/system/activate-configuration.sh b/system/activate-configuration.sh
index 7054a40e5c4..addeeb2f820 100644
--- a/system/activate-configuration.sh
+++ b/system/activate-configuration.sh
@@ -91,7 +91,9 @@ if test -z "@readOnlyRoot@"; then
 
     mkdir -p /nix/etc/nix
     cat > /nix/etc/nix/nix.conf <<EOF
+# WARNING: this file is generated.
 build-users-group = nixbld
+build-max-jobs = @maxJobs@
 EOF
 
     chown root.nixbld /nix/store
diff --git a/system/options.nix b/system/options.nix
index 95345faf47c..10f6032ad1a 100644
--- a/system/options.nix
+++ b/system/options.nix
@@ -663,6 +663,19 @@
   }
 
   
+  {
+    name = ["nix" "maxJobs"];
+    default = 1;
+    example = 2;
+    description = "
+      This option defines the maximum number of jobs that Nix will try
+      to build in parallel.  The default is 1.  You should generally
+      set it to the number of CPUs in your system (e.g., 2 on a Athlon
+      64 X2).
+    ";
+  }
+
+  
   {
     name = ["security" "setuidPrograms"];
     default = ["passwd" "su" "crontab" "ping" "ping6"];
diff --git a/system/system.nix b/system/system.nix
index 02410482e8e..b68ca907032 100644
--- a/system/system.nix
+++ b/system/system.nix
@@ -224,13 +224,13 @@ rec {
     readOnlyRoot = config.get ["boot" "readOnlyRoot"];
     hostName = config.get ["networking" "hostName"];
     setuidPrograms = config.get ["security" "setuidPrograms"];
+    maxJobs = config.get ["nix" "maxJobs"];
 
     path = [
       pkgs.coreutils pkgs.gnugrep pkgs.findutils
       pkgs.glibc # needed for getent
       pkgs.pwdutils
     ];
-
   };