Changes in the testosterone to dihydrotestosterone ratio in plasma and testes of maturing rabbits

Testosterone (T) and dihydrostestosterone (DHT) were radioimmunologically assayed in the testes and plasma of rabbits at 1, 10, 20, 40, and 60 days of age and at 3, 4, 5, and 8 months. In the testis (nanograms per 100 mg testis) and plasma (picograms per ml), both hormones are low at birth, reach their maxima between 60--90 days, and then decline and remain low after 120 days. In the testes, the T:DHT ratio is very high from birth (4.2 +/- 0.4) to 60 days (8.8 +/- 1.1). A significant decrease (P less than 0.001) occurs between 60 (8.8 +/- 1.1) and 90 (1.2 +/- 0.1) days. The T:DHT ratio is always less than or equal to unity after 90 days. From 1--60 days, T is the dominant testicular hormone, while the levels of DHT are greater than or equal to those of T after 60 days. The mean value of the plasma T:DHT ratio is always greater than unity from birth (1.7 +/- 0.4) to adulthood (2.6 +/- 0.7). The dominant circulating hormone, at all ages, is T. Our results show that sexual maturation in rabbits is characterized by an inversion of the T:DHT ratio in the testes but not in the plasma.