Isatin is an endogenous indole which has been shown to counteract some of the effects of atrial natriuretic peptide (ANP) both in vitro and in vivo. The present study was designed to determine whether it could antagonise in vivo effects of the related peptides brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). The model used was consolidation of memory in a one-trial step-through passive-avoidance paradigm in the rat. Previous studies have shown that all three peptides (1 microg intracerebroventricular) can consolidate such learning and increase latency to entry a dark box. Isatin was given intraperitoneally at doses of 5, 10 and 50 mg/kg before the peptide, or a saline control. Both BNP and CNP significantly increased the latency of entry. Isatin alone had no effect. Isatin reduced the effect of both BNP and CNP; this was significant for its effect on BNP at 50 mg/kg and on CNP at both 10 and 50 mg/kg. These results show that isatin can inhibit behavioural effects of BNP and CNP as well as ANP.