Androgens in the fetal guinea-pig after maternal infusion of radioactive testosterone

Testosterone was measured in plasma pools from male and female fetal guinea-pigs between the ages of 30 and 55 days of pregnancy. Between days 33 and 36 the testosterone concentration in the plasma of males (1.4 ng/ml) was several times higher than that found at other ages or that measured in female fetuses. After infusion of tritiated testosterone for 2 h into pregnant guinea-pigs at day 36 of pregnancy, high levels of testosterone and androstenedione were found in maternal plasma. Nevertheless, tritiated testosterone and androstenedione could hardly be shown in the fetuses. Similar large differences in plasma progesterone levels appeared to exist between the maternal and the fetal circulation. Therefore, only a very small fraction of these steroids can penetrate from the maternal circulation into that of the fetus. This finding might be explained by the large difference in androgen-binding capacity between maternal and fetal plasma, as was shown by equilibrium dialysis.