1. The effect on platelet aggregation of glyceryl trinitrate in the presence of cultured vascular smooth muscle cells was determined turbidometrically. U46619 (a thromboxane mimetic) was used as agonist and experiments were performed in the presence of aspirin. Inorganic nitrite production from glyceryl trinitrate by vascular smooth muscle cells was also measured, to provide an indirect index of nitric oxide synthesis. 2. The combination of vascular smooth muscle cells together with glyceryl trinitrate, at concentrations that had little effect individually, profoundly inhibited platelet aggregation. 3. The inhibitory effect on platelet aggregation of vascular smooth muscle cells together with glyceryl trinitrate was markedly attenuated by haemoglobin, an inhibitor of nitric oxide. 4. These results show that vascular smooth muscle cells inhibit platelet aggregation when exposed to glycerol trinitrate and suggest that this is due to generation of nitric oxide from glyceryl trinitrate by vascular smooth muscle.