Testosterone (T). 5α-dihydrotestosterone (DHT), 5α-androstane-3α, 17β-diol (3α-diol), and 5α-androstane-3β,17β-diol (3β-diol) have been measured in the plasma, ventral prostate, and testes of Sprague-Dawley rats with a specific radio-immunoassay, at birth, and then weekly until the 12th week.
In plasma, the T concentration is ∼ 0.3 ng/ml at birth and up to the 4th week; thereafter it increases steadily and reaches the adult level of ∼ 3 ng/ml by the 8th week. In male foetuses of 18 days gestation, the plasma concentration is 0.6 ng/ml. The 5α-metabolites taken together are at a higher concentration than that of T until the 7th week, and each one, after a broad maximum between 5–9 weeks, decreases promptly by the 10th week. The larger formation of 5α-metabolites from T in young rats has been confirmed by injection of the hormone to castrated males, which showed relatively more conversion at 5 than at 10 weeks.
In the ventral prostate, DHT is already high at 4 weeks (11 ng/g), and remain between 7 and 15 ng/g until the 12th week. T concentration is also larger than in plasma at 4th week (2 ng/g) and becomes approximately equal to the plasma value at puberty. A sharp peak of 3α-diol and 3β-diol is observed at the 8th week.
In the testes, T concentration is high at birth (183 ng/g), then declines rapidly, and increases again from the 5th week onward in parallel with plasma concentration. The concentrations of 5α-metabolites per testis vary in parallel to those in plasma, with the exception of sustained elevation of 3β-diol.