In this study, we have investigated the release of nitric oxide from resting human platelets. Nitric oxide was detected and quantitated by either measuring the conversion of oxy-hemoglobin to met-hemoglobin or generation of nitrite and nitrate by the cells. Nitric oxide was released from both intact resting platelets and platelets activated by collagen. Nitric oxide release was proportional to platelet concentration, and was equivalent to approximately 4.5 +/- 0.6 pmol (or 2.8 +/- 0.3 pmol in the presence of prostaglandin I2) and 11.2 +/- 1.3 pmol nitric oxide released per minute per 10(8) cells at 37 degrees C for resting platelets and platelets activated by collagen, respectively. The generation of nitric oxide by resting platelets was linear with respect to time over a two hour period, while the release of nitric oxide from platelets following activation was transient and was linear for only the first 10 min, after which it slowed to completion at approximately 30 min. The release of nitric oxide was it slowed to completion at approximately 30 min. The release of nitric oxide was stimulated by L-arginine, but was inhibited by L-nitro-arginine methyl ester (L-NAME). The inhibitory effect of L-NAME could be reversed by addition of L-arginine. The release of nitric oxide from platelets was also partially inhibited by prostaglandin I2, prostaglandin E1, aspirin and EDTA. The amount of nitric oxide released from resting platelets compared with that released from endothelial cells suggests that platelet-derived nitric oxide may play a significant role in the maintenance of vascular tone and blood flow.