Because some baboons repeatedly infused with somatostatin died we reviewed available autopsy material. All six animals chronically treated with somatostatin displayed gross or microscopical pulmonary hemorrhage and increased hemosiderin in lung and liver whereas only one of six untreated animals had a similar abnormality. We therefore examined the hemostatic system in living baboons. Thrombocytopenia (mean platelet count of 84,000 per microliter) was noted in six of seven baboons chronically treated with somatostatin; platelet survival was normal. Clotting factors were unaffected. Fibrinogen concentration and survival were unchanged. The acute effects of intravenous somatostatin (0.8 micrograms per kilogram per minute for two hours) in previously untreated animals transiently decreased platelet count, reduced retention of platelets on glass-bead columns and inhibited aggregation induced by ADP, collagen and epinephrine. Bleeding times were not prolonged. Somatostatin added to platelet-rich plasma in vitro was without effect. These data suggest that prolonged administration of somatostatin should be undertaken with caution.