Using specific antibody raised against renal renin, we have documented that the majority of the uterine renin-like activity in gravid and nongravid uteri is immunoreactive renin. To characterize its physiochemical properties, we obtained highly purified uterine renin by two affinity chromatographic steps, pepstatin and antirenin. Uterine renin has a pH optimum of 6, an apparent mol wt of 38K, and a Km of 1.7 Î¼M for homologous substrate. These properties are identical to those of renal renin and are not influenced by the pregnant state. In the basal state, an inactive form of the uterine enzyme constitute 55 Â± 10% of the total uterine renin. During pregnancy, active renin increased 40-fold as inactive renin fell to 4 Â± 3% of the total renin concentration. The renal renin concentration fell as plasma renin increased during pregnancy. These data suggest that the increased uterine renin concentrations during pregnancy are probably due to increased local production and conversion of renin precursor to the active enzyme. This stimulation of the uterine renin level appears to be independent of renal renin.