Bilateral injections of either neurotensin (NT; 0.3, 1 or 5 micrograms in 1 microliter artificial CSF) or haloperidol (HA; 2.5 or 5 micrograms in 1 microliter 0.3% tartaric acid) into nucleus accumbens of rats markedly diminished the forward locomotion and rearing induced by d-amphetamine (AM; 2 mg per kg, IP). Neither NT nor HA affected the insomnia or sniffing component of AM arousal. Isovolumetric intra-accumbens injections of artificial CSF or the endogenous decapeptide, luteinizing hormone-releasing hormone (LHRH; 3 micrograms), did not affect AM behaviours. Since intra-accumbens injections of NT (1 microgram) or HA (2.5 micrograms) neither altered forward locomotion or rearing observed in untreated rats placed in an open field nor a variety of reflex activities, the observed effects of NT and HA in AM-treated rats were probably not due to impaired motor function per se. In contrast, NT does not produce neuroleptic-like effects when injected into nucleus caudatus; HA (5 micrograms) blocked stereotyped sniffing, licking, biting and head bobbing observed after AM (5 mg per kg, IP), but NT (3 or 5 micrograms) did not. Since NT and dopamine are present in substantial quantities in the nucleus accumbens, NT may act in the nucleus accumbens to modulate dopaminergic function.