We examined the effects of nitric oxide (NO)/endothelium-derived relaxing factor (EDRF) on platelet surface glycoproteins (GP). As determined by flow cytometry, in both a washed platelet system and platelet-rich plasma, the EDRF congener (S-nitroso-N-acetylcysteine) markedly inhibited both the thrombin-induced and the (stable thromboxane A2 analogue) U-46619-induced upregulation of P-selectin (alpha-granule protein), CD63 (lysosomal protein), and the GPIIb-IIIa complex (fibrinogen receptor) but minimally inhibited downregulation of the GPIb-IX complex (von Willebrand factor receptor). The inhibitory effects of EDRF were markedly reduced in whole blood or by the addition of washed erythrocytes. Platelets in whole blood were still responsive to guanosine 3',5'-cyclic monophosphate (cGMP), as shown by complete inhibition of P-selectin upregulation by the stable analogue N6,2'-O dibutyryl cGMP. These data suggests that 1) cGMP negatively regulates the platelet surface expression of P-selectin, CD63, and the GPIIb-IIIa complex but not the platelet surface expression of the GPIb-IX complex and 2) hemoglobin within erythrocytes inhibits the effects of EDRF/NO on platelet surface glycoproteins.