Studies were performed to determine whether endothelium-derived relaxing factor (EDRF) can inhibit platelet aggregation within the vascular lumen, and if so, whether the inhibition persists in the presence of red blood cells (RBCs). Canine femoral arteries mounted in an organ bath were perfused with physiologic saline solution to which acetylsalicylic acid was added to block prostacyclin formation. During contraction with phenylephrine, addition of acetylcholine to the perfusing solution to evoke EDRF release relaxed the vessel wall. Washed human platelets labeled with 14C-5-hydroxytryptamine were added to the perfusing solution, and activated by thrombin infused via a branch vessel. The perfusate was collected downstream and centrifuged; the fraction of 14C-5-hydroxytryptamine appearing in the supernatant reflected the degree of platelet activation. Stimulation of EDRF release with acetylcholine inhibited 14C-5-hydroxytryptamine release. Hemoglobin (Hb) (10(-5) mol/L) blocked vascular relaxation and platelet- inhibition. RBCs at a hematocrit of 10% (treated with echothiophate to block erythrocyte cholinesterase) did not prevent relaxation but reversed the platelet inhibition. Lower hematocrits did not completely block the inhibition. Thus, erythrocyte Hb may modulate the inhibition of intraluminal platelet aggregation by EDRF.