Compensatory responses tending to prevent a thrombotic state in rats fed a high lipid diet have been investigated. Platelet membranes from these animals had an increased cholesterol content but the membrane fluidity was found to be within values nearly normal. A decrease in phosphatidylethanolamine was noted. These changes may maintain normal platelet sensitivity to aggregating agents. In fact, platelets from hyperlipidemic rats were hypersensitive to thrombin, but not to adenosine diphosphate. In addition, platelets were apparently able to correct, at least in part, the stated hyperactivity of hyperlipidemic plasma to coagulate, as shown by thrombelastographic tests in both platelet-rich plasma and plasma from hyperlipidemic rats. Finally, thrombelastographic features of whole blood from these animals were found to be normal. This suggests an important role of blood cells in compensating plasma hyperactivity to coagulate during hyperlipidemia.