Morphine (2.5 mg/kg, i.p.) and MK-801 (0.05 mg/kg, i.p.) each enhanced the rewarding impact of electrical stimulation of the medial forebrain bundle, causing small but reliable parallel leftward shifts of the functions relating response rate to stimulation frequency. Administration of MK-801 and morphine together caused a profound leftward shift in the functions. This effect was not due to sensitization to either drug, and suggests that disruption of glutamatergic function can potentiate the rewarding impact of opiates.