1. Full inhibition of thrombin-induced platelet aggregation was elicited by the least maximal platelet inhibitory concentrations of nitric oxide (NO; 7 +/- 1 microM) or NO-donors which included sodium nitroprusside (NaNp; 80 +/- 13 microM) 3-morpholinosydnonimine (SIN-1; 3 +/- 0.1 microM) or endothelial cells (EC; 2.36 +/- 0.12 x 10(5) added 1 min before thrombin. Oxyhaemoglobin (oxyHb; 10 microM) administered 30s to 10 min after stimulation with thrombin caused a time-dependent reversal of the inhibition induced by these agents. OxyHb was ineffective when these agents were co-incubated with the non-selective phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX, 0.05 mM). 2. OxyHb did not reverse the platelet inhibition with IBMX (0.2 mM) or that caused by a selective guanosine 3'; 5'-cyclic monophosphate (cyclic GMP) phosphodiesterase inhibitor 2-O-propoxyphenyl-8-azapurin-6-one, (M & B 22948; 20 microM). In addition, oxyHb did not reverse the inhibition with iloprost (1 nM) which inhibits platelet aggregation through stimulation of adenylate cyclase. 3. The inhibition of platelet aggregation by NO (7 +/- 1 microM) or NaNp (80 +/- 13 microM) was accompanied by a 13 fold increase in cyclic GMP levels occurring within 15 s of addition of these agents. In the continued presence of NO or NaNp, the reversing effect of oxyHb given 1 min after thrombin was associated with a pronounced decrease in cyclic GMP levels. 4. We conclude that the inhibition of platelet aggregation by activators of guanylate cyclase depends in the first few minutes on continuous stimulation of the enzyme in order to maintain intracellular concentrations of cyclic GMP, except when its breakdown is inhibited. 5. The addition of agents such as oxyHb after the inhibition of platelet aggregation offers another way of investigating the biochemical changes involved in maintaining platelets in an inactive state.