Radioimmunoassay of testosterone in late fetal and newborn rabbit plasma, gonads and adrenal glands]

A radioimmunoassay was used for measuring testosterone in the plasma, gonads and adrenals of 28, 29, 30 and 31-day-old rabbit fetuses of both sexes and newborns. A marked sex difference was shown in the concentrations of testosterone in plasma and in gonads whereas in adrenals the levels of testosterone were low in both sexes (34 to 147 pg/10 mg). In male fetuses, plasma testosterone levels increased from the 28th (133 +/- 20 pg/ml) to the 31st day (361 +/- 119 pg/ml) of intrauterine life, reaching then the values observed in the newborns (387 +/- 73 pg/ml). Plasma from males, on the other hand contained, at all stages studied, significantly more testosterone than plasma from female fetuses (21 +/- 6 to 41 +/- 11 pg/ml) and female newborns (42 +/- 6 pg/ml). In the same way, fetal testicular testosterone concentrations varying from 1 382 +/- 218 to 2 317 +/- 333 pg/10 mg were similar to those measured in the newborns (1 940 +/- 304 pg/10 mg) and significantly higher than fetal (13 to 34 pg/10 mg) or neonatal (44 pg/10 mg) ovarian concentrations. These results showed at evidence the endocrine activity of the fetal testis during this period.