The ability of morphine and amphetamine to potentiate brain stimulation reward was studied in rats with monopolar electrodes in the medial forebrain bundle. Animals received seven daily injections of morphine (3 mg/kg, s.c.) or saline followed 10 min later by either DNQX (100 mg/kg, i.p.) or its vehicle. On the 8th day the self-stimulation (SS) response was examined 60 min or 30 min after the administration of either morphine (3 mg/kg, s.c.) or amphetamine (1.5 mg/kg, s.c.), respectively. It was found that the subchronic administration of morphine sensitizes animals to subsequent amphetamine-, but not morphine-induced activation of SS, which may be prevented by the coadministration of DNQX, an antagonist of the non-NMDA subtype of glutamate receptors.