1. Determinations of plasma renin activity (PRA), plasma renin concentration (PRC) and renin substrate (RS) were made on single plasma samples from thirty-four women at various stages of pregnancy. Comparisons were made with non-pregnant control subjects. 2. All three variables were elevated above controls throughout pregnancy, but whereas the mean value of PRA was equally high between the first and the second halves of pregnancy, PRC was higher in the first and RS higher in the second half of pregnancy. 3. Kinetic studies indicated that, in spite of high concentrations of RS in late pregnancy, the renin-renin substrate reaction remains substrate dependent when proceeding at physiological pH values. 4. No differences in enzyme-substrate affinity were detected between the plasmas of pregnant and non-pregnant women. 5. In each of three deliveries RS was lower in the foetus than in the mother but PRA and PRC did not display consistent gradients. 6. PRC in uterine venous blood was slightly lower (P = 0·05) than in peripheral blood during caesarian section at term. 7. On deriving PRC from PRA by using the Michaelis-Menten equation, a physiologically inactive component in maternal PRC became apparent. It is suggested that this component is activated by the acid treatment used in the direct PRC methodology. Its concentration is highest in early pregnancy and circumstantial evidence suggests that it originates from foetal chorion.