Serum concentrations of LH, FSH and prolactin were measured in neonatal rats. Intact male pups consistently showed low levels of FSH and LH, but serum levels of both these hormones were increased several-fold one day after castration. The response to castration was greater in 5- to 10-day-old males as compared to younger animals. Indeed, the gonadotropin levels in 5- to 10-day-old castrates were about as high as those seen in adult males which had been castrated for one day. Female pups displayed variable serum LH levels, but both LH and FSH levels were considerably higher than those observed either in males of the same ages or in adult diestrous females. Serum prolactin levels were low in pups of both sexes as compared to adult rats.
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