This study was undertaken to determine whether the nitric oxide/platelet cyclic guanosine monophosphate (NO/cGMP) pathway might be used to reduce platelet activation by artificial surfaces. Because serotonin release (SR) is a platelet activation indicator, rabbit platelets in their own plasma (PRP) were labeled with 3H-serotonin. Labeled PRP was incubated with glass beads for 5-10 min, at 37 degrees C with gentle agitation, and SR was measured. PRP pretreatment with NO gas or nitroprusside + N-acetylcysteine inhibited SR 50%. Dose response studies indicate the existence of an optimal NO concentration above which its inhibitory effect is diminished. The guanylate cyclase inhibitor methylene blue attenuates the NO effect, implicating cGMP in NO mediated inhibition of surface induced platelet activation. Adult pigs were supported on a membrane oxygenator in an in vitro model of cardiopulmonary bypass (CPB). Introduction of NO gas into the oxygenator sweep gas at 500 ppm reduced platelet adherence to the oxygenator surfaces, increased circulating platelet counts, and decreased the rate of platelet aggregation (whole blood impedance platelet aggregometry) compared with the results of the control animals. Indications of NO toxicity were seen when the NO flow rate was increased to 1,000 ppm. These studies support the hypothesis that NO reduces platelet activation by artificial surfaces in clinical devices.