The cellular and subcellular distribution of 3H-progesterone or its metabolites was studied in the oviduct, uterus, vagina, liver and diaphragm in 30-day-old estrogen-primed and non-primed ovariectomized guinea pigs using drymount autoradiography. In autoradiograms from estrogen-primed and untreated ovariectomized guinea pigs, prepared 15 min or 30 min after the intravenous injection of l,2,6,7-3H-progesterone, a nuclear concentration of radioactivity was observed in the oviduct in the luminal epithelium, connective tissue and muscularis; in the uterus: in the luminal and glandular epithelium, stroma and muscularis; and in the vagina: in the basal layers of the stratified squamous epithelium, connective tissue and interlacing bundles of smooth muscle. In the nonprimed guinea pigs, the nuclear concentration of radioactivity was less pronounced with exception of the glandular cells. In the diaphragm, no selective cellular and subcellular concentration of radioactivity existed. In the liver, similar to diaphragm, no selective nuclear concentration was observed; however, the cytoplasm of hepatocytes in general contained a relatively high amount of radioactivity. The biliary system showed concentration of radioactivity at cell interphases (bile canaliculi), in canals of Hering and in the lumena of bile ducts. While the epithelium of bile ducts appeared free of radioactivity, certain cells, probably lipocytes, showed a selectively high cytoplasmic concentration. Pretreatment with progesterone, but not with cortisol, reduced the nuclear concentration of radioactivity in oviduct, uterus and vagina. When dihydroprogesterone was injected prior to 3Hprogesterone, approximately a 45% reduction in the nuclear uptake of radioactivity was observed. The autoradiographic results suggest the existence of progestin target cells in reproductive organs and in the liver. (Endocrinology94: 1116, 1974)