The distribution of 125I-labeled ovine prolactin was studied in mice and rats after an intravenous injection by using whole-body and microautoradiography. In the whole-body autoradiography the labeled hormone rapidly accumulated in liver and kidney, but additionally some labeling was noted in the ventral prostate, seminal vesicles and mammary glands. In microautoradiography the grains were localized almost exclusively in the proximal tubular cells of the kidney. In the liver the labeling was more diffuse over both the parenhymal and the KupfTer cells. In the ovary the labeling was weak and mainly localized over the corpus luteum. Only minor differences were encountered in the intensity of labeling during the sexual cycle, pregnancy and lactation. The mammary epithelium showed some labeling during pregnancy and lactation. In the testis the grains were localized around the Leydig cells with no indication of tubular distribution. The ventral prostate showed a clear labeling of the epithelium. With increasing time after the injections the grains were rapidly localized toward the prostatic acinar lumen. Only a weak labeling of the preputial glands and seminal vesicles was observed. The role of prolactin as a synergist in the hormonal control of a variety of target organs was discussed.