Histamine, a major constituent of the amine-storage organelles in pig platelets, is taken up by intact platelets in only trace amounts under conditions where 70% of 14C-serotonin is accumulated. Thrombin caused the release of 70-90% of endogenous histamine but only 5-10% of the newly absorbed 3H-amine; however, after 18 hr 30% of the 3H-amine could be specifically released by thrombin. Isolated storage organelles accumulated histamine in a reserpine-sensitive, ATP-dependent manner but at a rate 80-100-fold less than serotonin uptake. Incubation of intact platelets with 1 mM serotonin until amine uptake was saturated caused no changes in platelet histamine content. Similarly, loading of isolated storage organelles with 1 mM histamine or 1 mM serotonin did not affect the levels of the other amine. These results suggested that the storage of each amine is independent of the other. Histidine decarboxylase was not detected in platelet lysates. Since platelets have a short half-life (1-2 weeks) and pig plasma levels of histamine are higher than in other animals, it is concluded that most of the histamine in the storage organelles is probably accumulated in the platelet precursor, the megakaryocyte, either by slow uptake or by synthesis.