The concentrations of LH in serum have been measured by radioimmunoassay in golden hamsters maintained in an environment with controlled lighting (on at 5 AM; off at 7 PM, CST). The maximal concentration of LH in serum of cycling hamsters occurred between 3 and 4 PM on the proestrous day, at which time the LH concentration increased as much as 50- to 100-fold within a short time. The pattern of LH release in the cycling hamster is very similar to that in the rat. Ovulation occurs 9-10 hr after the LH surge. In contrast to intact female hamsters during diestrus in which the concentration of LH in serum is quite low, female hamsters ovariectomized during diestrus have markedly increased serum LH levels within 3.5 hr following removal of the ovaries. Intact males showed great variability in serum LH levels, and the average LH concentration in intact male hamsters was much higher than that observed in diestrous females. Serum LH concentrations in males were noticeably elevated 24 hr after removal of the testes. LH levels in the serum of hamsters which had been gonadectomized for as long as 39-50 days were rarely as high as those observed during the proestrous LH peak.
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