The dopamine D3 receptor (DRD3) mediates expression of conditioned effects of psychostimulants, but conflicting results have been obtained with opiates. In a conditioned place preference (CPP) procedure, morphine increased the time spent in a compartment previously paired with drug injection. CPP was obtained at morphine doses of 16 and 32 mg/kg in wild-type (drd3+/+) mice and 8, 16 and 32 mg/kg in DRD3-knockout (drd3-/-) mice. BP897, a DRD3-selective partial agonist, inhibited the expression of morphine-CPP in drd3+/+, but not drd3-/- mice. BP 897 reduced brain regional activation, measured by c-fos imaging after the CPP test session, in the somatosensory cortex of drd3+/+, but not drd3-/- mice. These results confirm the role of DRD3 in the expression of conditioned effects of morphine and the participation of the somatosensory cortex in these effects.