Rabbit aortic strips were arranged in a Vane's cascade and superfused with Krebs buffer which contained phenylephrine hydrochloride (100 nM) and indomethacin (5 microM). EC50 (nM) for vasorelaxant action of NO-donors on rabbit aortic strips were as follows: glyceryl trinitrate--64.0, sodium nitroprusside--124.5, S-nitroso-N-acetylpenicillamine--305.7, SIN-1 (metabolite of molsidomine)--510.0. On the other hand IC50 (microM) for antiaggregatory action of NO-donors in human platelet rich plasma aggregated with threshold concentrations of collagen (0.5-1 micrograms/ml) were as follows: S-nitroso-N-acetylpenicillamine--7,1, SIN-1--13.6, sodium nitroprusside--28.0. Glyceryl trinitrate did not inhibit platelet aggregation at concentrations up to 1 mM. Sodium nitroprusside was more specific (EC50/IC50 = 0.0043) than SNAP and SIN-1 (EC50/IC50 = 0.043 and 0.037, respectively) as a vasorelaxant than as a platelet suppressant. Thus relative potencies of four c-GMP stimulants as vasorelaxants and platelet-suppressants differed considerably and several possible explanations for these differences were put forward.