diff --git a/modules/services/x11/display-managers/default.nix b/modules/services/x11/display-managers/default.nix
index 2261d417d04f..cdfd271ee65c 100644
--- a/modules/services/x11/display-managers/default.nix
+++ b/modules/services/x11/display-managers/default.nix
@@ -80,6 +80,8 @@ let
 
       source /etc/profile
 
+      ${cfg.displayManager.sessionCommands}
+
       # Allow the user to setup a custom session type.
       if test -x ~/.xsession; then
           exec ~/.xsession
@@ -164,6 +166,16 @@ in
         apply = toString;
       };
 
+      sessionCommands = mkOption {
+        default = "";
+        example =
+          ''
+            xmessage "Hello World!" &
+          '';
+        type = types.string;
+        description = "Shell commands executed just before the window or desktop manager is started.";
+      };
+
       session = mkOption {
         default = [];
         example = [
diff --git a/modules/virtualisation/virtualbox-guest.nix b/modules/virtualisation/virtualbox-guest.nix
index 9f215e816563..04fca1eb4e98 100644
--- a/modules/virtualisation/virtualbox-guest.nix
+++ b/modules/virtualisation/virtualbox-guest.nix
@@ -60,6 +60,11 @@ in
         InputDevice "VBoxMouse"
       '';
     
+    services.xserver.displayManager.sessionCommands =
+      ''
+        PATH=${makeSearchPath "bin" [ pkgs.gnugrep pkgs.which pkgs.xorg.xorgserver ]}:$PATH \
+          ${kernel.virtualboxGuestAdditions}/bin/VBoxClient-all
+      '';
   };
 
 }