The role of endogenous glutamate neurotransmission within the nucleus accumbens in the modulation of intravenous (i.v.) cocaine and heroin self-administration in rats was analyzed. APV (2-amino-5-phosphonovaleric acid), a blocker of glutamate receptors of the N-methyl-D-aspartate (NMDA) type, was microinfused within the nucleus accumbens of the ventral striatum of rats trained to lever press for i.v. cocaine or heroin self-administration. APV, at the dose of 1.5 and 3.0 micrograms/side, reduced the rewarding value of cocaine while it left heroin self-administration unaffected. These results suggest that integrity of NMDA receptor function within the nucleus accumbens may be of importance for the maintenance of i.v. cocaine, but not heroin self-administration in rats.