Serum LH and FSH, testis weight (TW), ventral prostate weight (VPW) and testicular histology were assessed in the immature male rat from age 21 to 91 days. Intact, castrate and cryptorchid male rats were studied at biweekly intervals. In intact animals, TW and VPW increased with age; mature sperm were first observed at 49 days. At 21 days LH was 65% of adult levels and increased progressively with age. In contrast, FSH in immature rats was 2-3 times higher than adult levels. FSH was maximal at 35 days of age and fell with the appearance of mature sperm in the tubules. This fall in FSH and the development of spermatogenesis were prevented by cryptorchidism. FSH values in the cryptorchid rat continued to rise until 49 days, at which time VPW and LH were not different from the controls, indicating unaltered Leydig cell function. Two weeks following castration of 21-day-old rats, LH and FSH were 5 times higher than intact adult levels. Marked destruction of germinal epithelium by cryptorchidism failed to elevate serum FSH to castrate levels. These data indicate that, in the immature male rat: 1) a dynamic hypothalamic-pituitarygonadal relationship exists; 2) the control systems for LH and FSH secretion differ; 3) inhibition of FSH to adult levels is coincident with the development of mature spermatogenesis; and 4) while inhibition of FSH secretion appears to be in part related to germinal maturation, it must be in part affected by gonadal hormones.
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