Variations in serum FSH, LH and testosterone levels in male rats from birth to sexual maturity.

Serum LH, FSH and testosterone concentrations were measured by radioimmunoassays in male Sprague-Dawley rats from birth to 80 days of age. The levels of FSH were significantly elevated during the first 5 days of postnatal life. An abrupt decline in FSH concentrations occurred during this period, from levels of 800 ng/ml on day 1 to levels of 300 ng/ml on Day 6. Subsequently, FSH levels fluctuated widely until about Days 30 to 45, when a secondary peak of FSH was observed. Thereafter, a decline in FSH levels to those found in adult rats occurred. This decline in FSH levels appears to coincide with the first release of mature spermatozoa from the germinal epithelium in the testis. During the first 30 days of postnatal life, LH and testosterone values appeared to be inversely related to each other and an LH peak and a nadir of testosterone levels was observed between Days 6 and 14 at time corresponding to regression of the fetal generation of interstitial cells. A parallel rise in LH and testosterone levels occurred from Days 30 to sexual maturity and corresponded to the development of the adult generation of intestitial cells.