Accumulation of the weak base aminopyrine was used as an index of acid secretory activity in rat isolated parietal cells. The nitric oxide (NO) donor, S-nitroso-N-acetyl-penicillamine (SNAP) caused a dose-dependent inhibition of aminopyrine accumulation (half-maximally effective concentration 247 microM) which was accompanied by an increase in guanosine 3',5'-cyclic monophosphate (cGMP) but no decrease in cell viability (trypan blue), glucose oxidation or adenosine 3',5'-cyclic monophosphate (cAMP) content. Oxyhaemoglobin (37 microM), which scavenges NO, significantly reduced the inhibitory effect of SNAP (1 mM). Prior exposure of intact cells to SNAP also reduced aminopyrine accumulation in response to ATP in permeabilised cells, an effect prevented by Rp-8-bromoguanosine 3',5'-monophosphorothioate, which inhibits activation of cGMP-dependent protein kinase, but not by the Sp-isomer. NO thus inhibits secretory activity in rat parietal cells by a specific interaction that may involve cGMP.