Mean (±SEM) seconds per minute spent on the grid floor during floor choice testing for DARPP-32 KO mice and WT mice. Test 1 occurred after four conditioning trials, test 2 after two additional conditioning trials, and test 3 after two more additional conditioning trials. Grid+ groups had previously received pairings of the grid floor with drug treatment (and hole floor with saline), whereas Grid− groups had previously received pairings of the grid floor with saline (and hole floor with drug treatment). Conditioned place preference (*p < 0.05) is shown when time spent on grid floor by the Grid+ group exceeds time spent on the grid floor by the Grid− group.