In slices of non-pregnant rabbit uterus, renin was gradually inactivated during incubation in vitro at 37 degrees C, for 44 hours. The percentual rate of inactivation (40 per cent/44 hours) was, however, only about half of that found in uterine slices from pregnant and post-partum animals despite the fact that the endogenous renin concentration was much lower in the non-pregnant uteri. In control experiments with kidneys with the same high renin content as in the pregnant uteri only about 30 per cent of the renin was inactivated in 44 hours.