1. In unanaesthetized preparations 6-14 days after catheterization, the concentration of renin substrate (AI 212 +/- 20 ng/ml. S.E. of mean) in the plasma of foetal lambs during the last third of gestation was significantly less than that of paired samples of maternal plasma (AI 350 +/- 27 ng/ml.). 2. During the first 5 days following catheterization the concentration of renin substrate in maternal plasma was lower than subsequently but was raised if the uterus contained a nephrectomized foetus. 3. The concentration of renin substrate in the plasma of intact foetal lambs without nephrectomized twins varied little in the post-catheterization period and later; the concentration of substrate in nephrectomized foetal lambs averaged threefold that of intact lambs and a similar concentration of substrate was found in intact twins of such lambs in the immediate post-catheterization period. 4. It is suggested that the ratio of foetal/maternal plasma renin activity understates their relative enzyme activity since foetal substrate concentration is so considerably less than maternal.