The concentrations of progesterone (P), estrone (E1) and estradiol-17β (E2β) in the peripheral plasma of rabbits during the course of pregnancy have been determined by radioimmunoassay. The mean concentration of P increased from 5.3 ng/ml on day 3 to 17-19 ng/ml on days 12-15 and declined slowly thereafter until term. The mean concentration of E1 remained between 20-35 pg/ml throughout gestation. The levels of E2β showed fluctuations with peaks on day 6 (83 pg/ml), day 15 (59 pg/ml) and day 30 (48 pg/ml). The concentrations of P and E2β rose and fell similarly between day 9 and day 24. Thereafter, E2β rose but P declined. The ratio, P/E2β, fell sharply from 480 to 140 between days 27 and 30. (Endocrinology93: 971, 1973)