Abstract. Testosterone (T), dihydrotestosterone (DHT), androstenedione (Δ4) and dehydroepiandrosterone (DHA) were measured by radioimmunoassay in peripheral plasma, uterine vein plasma, ovary and placentae within the last 3–4 days of pregnancy in Sprague-Dawley rats. In the uterine vein, T and Δ4 levels were significantly higher than those reported in the peripheral plasma at any particular day of late pregnancy. In peripheral plasma as in uterine vein plasma Δ4 and T gradually decreased from day 20 to term (−63% for Δ4 and approximately −45% for T) whereas DHT concentrations were unchanged. This decline is concomitant with a similar pattern of change in the placentae; indeed placental concentrations of Δ4 and T decreased significantly from day 19 to 22 (−63 and −79%, respectively) and were positively correlated with the levels of both androgens in uterine venous and peripheral plasma. Thus it was concluded that in the late phase of pregnancy in the rat circulating Δ4 and T originate from placentae. In contrast, ovarian androgen concentrations showed a sharp increase from day 21 to term: 58% for DHA, 49% for Δ4, 87% for T and 24% for DHT. It is suggested that these increasing concentrations of ovarian androgens could be related to the next ovulatory cycle.