In rabbits, a clear microscopic image of individual, fluorescent blood platelets flowing in a mesenteric microvessel could be obtained over the whole cross-sectional area of the vessel, following an intravenous injection of a solution containing 5-15 mg of the fluorochrome Acridine Red. Most of the circulating platelets were labeled. Activation of the platelets by the injected dye was not seen. A fall in platelet count or hematocrit following injection did not occur. Electron microscopy revealed no gross ultrastructural changes. In vitro, the dye reduced platelet aggregation in a dose dependent way. In vivo, aggregation and adhesion of platelets as induced by laser injury or transection of an arteriole was observed in all cases, even following multiple injections of Acridine Red. Primary hemostatic plug formation times as measured in mesenteric arterioles were normal after the first and second injection of 5 mg Acridine Red, but prolonged after subsequent injections. Bleeding times as measured on the ear were prolonged following injection of 15 mg of the dye. It is concluded that this labeling procedure allows the study of the rheological behavior of the platelets in vivo. Whether this technique can also be used to study functional platelet behavior in vivo, needs further investigation.