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 mo. In the testis (ng/100 mg testis) and plasma (pg/ml), both hormones were low at birth, reach at their maxima between 60-90 days and then declined and remain low after 120 days. In the testes the T:DHT ratio was very high from birth (4.2 .+-. 0.4) to 60 days (8.8 .+-. 1.1). A significant decrease (P < 0.001) occurred between 60 (8.8 .+-. 1.1) and 90 (1.2 .+-. 0.1) days. The T:DHT ratio was always less than or equal to unity after 90 days. From 1-60 days, T was the dominant testicular hormone, while the levels of DHT were 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. Sexual maturation in rabbits is characterized by an inversion of the T:DHT ratio in the testes but not in the plasma.