1. Basal and stimulated guanylate cyclase activity during ADP-induced human platelet aggregation in comparison with the actions of sodium nitroprusside (SNP) on platelets was investigated. 2. Sodium nitroprusside exhibited both ex vivo and in vitro antiplatelet effects, as assessed by inhibition of subsequent ADP-induced aggregation in platelet-rich plasma. A strong correlation between decrease in aggregation and increase in platelet guanylate cyclase activity in the presence of SNP was obtained. 3. When SNP was administered after the induction of aggregation, it caused acceleration of disaggregation (in reversible aggregation) and produced disaggregation (under conditions of otherwise irreversible aggregation) which was time-dependent. 4. Platelet aggregation was accompanied by a transient increase in platelet cyclic GMP content and guanylate cyclase activation by the nitric oxide (NO) donor SNP. Changes in guanylate cyclase activity were haem-associated and probably reflected saturation of enzyme by haem. 5. Maximal SNP disaggregating effect coincided with peak guanylate cyclase responsiveness to SNP. 6. The present investigation provides evidence that increased responsiveness of platelet guanylate cyclase to NO during aggregation facilitates disaggregation in the presence of SNP. Thus, availability of NO (endogenous or exogenous) at sites of incipient platelet aggregation in vivo may play a pivotal role regarding limitation of this process.