In order to examine the subcellular distribution of 111In in 111In- oxine-labeled human and rabbit platelets, we employed a hypothetical grain technique of EM autoradiography analysis. The results indicate that in the rabbit 111In was concentrated within the platelet dense bodies, particularly when the platelets had been labeled in a plasma- free system. Under comparable conditions of labeling, human platelets appeared to accumulate almost all the radiolabel within the cytosol. Using inhibitors of 5-hydroxytrptamine (5-HT) uptake, i.e., cloimipramine, ouabain, sodium fluoride, p-chloromercuribenzoate, and reserpine, we were unable to demonstrate an active uptake process in either species. Both collagen and thrombin were able to cause dose- dependent release of radioactivity from the labeled rabbit platelets only. In the case of collagen, this mimicked endogenous 5-HT release and was inhibited by indomethacin. These results and their implications are discussed.