Electron microscopic studies were performed on rabbit heart valves, and all four valves were found to be lined with a continuous basement membrane (BM). The valvular BM was liable to digestion by either trypsin or collagenase, but brief exposure to trypsin was suitable for desquamating endothelial cells, while leaving BM exposed and intact. Such preparations were used to determine their platelet reactivity with heparinized and citrated whole blood or with platelet-rich plasma. Although various conditions of exposure were used, including shaking and centrifugation, little or no platelet adhesion to the BM was observed. With similar conditions of exposure, preparations of collagen showed massive platelet adhesion accompanied by aggregation. This vascular BM was similarly nonreactive to platelets when endothelial cell removal was accomplished by mechanical methods. Rabbit valvular BM appears to be a poorly thrombogenic surface.