A group of vasoactive drugs, commonly used in vascular disease, have been shown to affect human platelet behaviour. Glyceryl trinitrate, propranolol, tolazoline, phentolamine, and intensain inhibit platelet aggregation in vitro, and both in vitro and in vivo prevent adenosine diphosphate and noradrenaline from altering platelet electrophoretic mobility. The theoretical and practical implications are discussed.