Effect of different doses of centrally administered brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) were examined in rats with respect to anxiolytic properties in an elevated plus-maze model. BNP in doses of 100, 200 and 400 ng, and CNP in doses of 100 and 200 ng abolished the normal preference for the closed arms of the maze, and increased the percentage time spent in the open arms; this is consistent with an 'anxiolytic-like' effect. Doses of 50 and 1000 ng BNP, and of 25, 50, 400 and 1000 ng CNP produced no behavioural effects in the elevated plus-maze model. Pretreatment with an alpha-adrenoreceptor antagonist or a muscarinergic cholinergic blocker, antagonized the effect of 200 ng BNP in the elevated plus-maze test. The 'anxiolytic-like' effects of a BNP were not modulated by a dopaminergic blocker, an alpha-adrenoreceptor antagonist, a GABA receptor antagonist, a 5-HT receptor antagonist or an opiate antagonist. The 'anxiolytic-like' effect of CNP was prevented by a dopamine receptor antagonist, or an alpha- or beta-adrenoreceptor blocker but not by a muscarinergic cholinergic blocker, a GABA receptor, a 5-HT receptor antagonist or an opiate receptor antagonist. These results suggest that multiple neurotransmitter system activation might be responsible for the BNP and CNP-induced 'anxiolytic-like' activity.