We studied the time of appearance of renin-immunoreactive cells in prenatal human kidneys by immunohistochemistry and transmission electron microscopy (TEM). Renin immunoreactivity was detectable as early as the 5th or 6th gestational week in the mesonephros and appeared at 8 weeks of gestation in the metanephros. In a 8 week old embryo "renin granules" were seen by TEM in the juxtaglomerular epithelioid granular cells of metanephric tissue. In the embryos delivered by prostaglandin-induced abortion, granular renin immunoreactivity was also present in the cells of the proximal convoluted tubule. Our results support the contention that the renin-angio-tension system is active during fetal life and indicates a role for renin even during the embryonic period. In addition, they document a morphologically detectable effect of prostaglandins on the distribution of renin in the kidney.