Male mice were raised in cohabitation with females from birth to 90 days. Testosterone was measured every 10 days in plasma and testes. Sex difference in body weight was related to the pre-pubertal increase of testosterone levels in males. The weight of the seminal vesicle was positively correlated with circulating testosterone levels between 1 and 40 days but not between 50 and 90 days

Testosterone concentrations in the plasma and testes were high at birth: 630 pg/ml and 58.0 ± 17.7 ng/100 mg; they subsequently decreased during the first days of life and remained low until day 20: 240 ± 110 pg/ml and 0.1 ± 0.03 ng/100 mg. The testosterone levels then increased rapidly between days 20 and 30 and especially between 30 and 40 reaching their maxima: 5770 ± 1720 pg/ml and 123.7 ± 18.3 ng/100 mg testis. This increase was transitory and testosterone levels fell after day 40. By 90 days, the testosterone levels, 440 ± 65 pg/ml and 43.2 ± 5.5 ng/100 mg testis, were comparable to those measured at birth. Plasma testosterone and age were positively correlated between 1 and 40 days, and negatively between 50 and 90 days.

The first fertile matings occurred at age 40 days.