Levels of testosterone in the plasma, gonads, and adrenals during fetal development of the rabbit.

Testerone (T) concentrations in the plasma, gonads, and adrenals were measured by radioimmunoassay in 188 male and 160 female rabbit fetuses. Determinations were performed daily from the 20th to the 31st day of gestation and were correlated with the maternal plasma concentration of T. The T content of both testes remained relatively constant from the 20th (3040 pg) to the 26th day (3940 pg), and subsequently decreased until the 31st day (1630pg). the concentration of testicular T fluctuated only slightly from the 20th (1040 pg/mg) to the 23rd day (783 pg/mg), and thereafter decreased until the 31st day (138 pg/mg). The T levels in plasma of males (132-361 pg/ml) were significantly higher than those of females (21-116 pg/ml). Plasma T levels in males were relatively constant and did not exhibit any rise, which is similar to observations of the testis during differentiation of the genital tract. Testosterone concentrations were low in the adrenals (3.5-12.3 pg/mg) of both sexes and in the ovaries (1.5-20.4 pg/mg) of fetuses. These data provide the first evidence for testicular T secretion at the time of genital differentiation in the rabbit.