The aim of this study was to examine the behavioural effects induced by stimulation of CCK receptors in the nucleus accumbens of the rat, which contains a high density of heterogenously distributed CCK-B receptors. The drug BC264 (Boc-Tyr(SO3H)-gNle-mGly-Trp-(NMe)Nle-Asp-Phe-NH2), a highly potent and selective CCK-B agonist, injected into the postero-median or antero-median n. accumbens did not modify the spontaneous alternation and exploratory behaviour observed in a Y-maze. In contrast, when administered into the antero-lateral part of the n. accumbens, BC264 (0.3-1.0 nmol) reduced alternation to chance level, without changing the number of arm visits. This effect was suppressed by the selective CCK-B antagonist: L365,260, but not by the selective CCK-A antagonist: MK329. The physiological relevance of this effect was supported by the similar responses induced, in the same concentration range, by the CCK8-like agonist, BDNL (Boc diNle28,31CCK7). These results emphasize the functional heterogeneity of the CCK network in the n. accumbens of the rat and the participation of the peptide in the expression of alternation behaviour, through stimulation of CCK-B receptors. They also show that the recently described anxiolytic effects, induced by CCK-B antagonists, do not seem to occur at this level.