Effects of reserpine in vivo and in vitro on rabbit platelets in citrated platelet-rich plasma and in suspensions of washed platelets have been studied. Administration of reserpine ( 5 mg kg ) intraperitoneally 18 hr before platelets were isolated caused inhibition of collagen-induced aggregation but not of aggregation induced by ADP or thrombin. Thrombin-induced aggregation was slightly enhanced. Platelets from reserpine-treated rabbits were less adherent than control platelets to collagen-coated glass surfaces or to the subendothelium of the rabbit thoracic aorta. Similar effects on aggregation were obtained when reserpine (0.2 to 10 μM) was added to suspensions of washed rabbit platelets as little as 2 sec before the addition of collagen. Collagen-induced release of nucleotides and [ 14C]serotonin from prelabeled washed rabbit platelets was not affected by the presence of reserpine, whereas thrombin-induced release was slightly enhanced. Inhibition by reserpine (2–10 μM) of platelet adherence to a collagen-coated surface or to the subendothelium was also observed within a time interval too short for the reserpine to have caused depletion of platelet granule contents. Thus, reserpine has an immediate effect on the plasma membrane of the platelets which is responsible for inhibition of platelet adherence to collagen and hence of collagen-induced aggregation. This inhibitory effect differs from a much slower effect of reserpine at the granule membrane which results in the depletion of the granule contents of serotonin and adenine nucleotides. The effect of reserpine is not abolished by washing and resuspending platelets that have been exposed to reserpine in vivo. By inhibiting the interaction of platelets with collagen, reserpine may interfere with one of the components of hemostatic plug and thrombus formation.