C 14 -serotonin is taken up and retained for long periods of time by uterus, heart, and spleen. The C 14 -amine is firmly bound by constituents in the cell sap of the uterus, by the blood elements in the spleen, and by a combination of these as well as by a small granule fraction in the heart. Reserpine, but not lysergic acid diethylamide, nor compound 48-80, can release bound C 14 -serotonin horn uterus and spleen. There is a greater concentration of C 14 -serotonin in the uterus during the diestrus phase of the estrous cycle than during estrus.