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Blood samples were withdrawn at 10-, 20-or 30-min intervals for 6-12 hr from 13 ovariectomized and 7 intact female rhesus monkeys bearing indwelling cardiac catheters and restrained in primate chairs. Plasma LH and GH concentrations were determined by radioimmunoassay. Twenty-four experiments in ovariectomized animals revealed a striking rhythmic pattern in plasma LH levels having a period of approximately 1 hr, which was asynchronous with variations in plasma GH concentrations when these occurred. The LH oscillations, which had an excursion of 40-92 % of the mean hormone concentration, occurred with a mean period of 75 min and a mode of 60 min. In a typical rhythmic sequence, LH levels increased from nadir to peak during one 10-min sampling interval and decayed exponentially for the next 60 min. The t } calculated from 19 such decay curves based on 10-min sampling intervals was 72 min, which is similar to the estimated tj of exogenous LH in intact females. Moreover, there was a significant positive correlation between the duration of the decay and the magnitude of the next peak. Such oscillations did not occur in intact females. It is suggested that these circhoral (about an hr) oscillations in plasma LH concentrations, occasioned by ovariectomy, represent pulsatile discharges of LH from the pituitary and probably account for the greater part of the elevated LH plasma levels seen in ovariectomized animals. (Endocrinology 87: 850, 1970) I N THE course of experiments performed in bilaterally ovariectomized rhesus monkeys, sampled at relatively frequent intervals, circulating levels of LH were observed to be markedly unstable (1). On detailed examination, sharp oscillations in plasma LH concentrations with a periodicity of approximately one hour were revealed. The following report describes this unexpected phenomenon. Materials and MethodsThirteen adult female rhesus monkeys (Macaco mulatto), weighing 4.6-6.2 kg, were bilaterally ovariectomized under pentobarbital anesthesia and utilized 2 weeks to 7 months after ovariectomy. Seven intact females in the same weight range were also studied.

Surges in luteinizing hormone secretion resembling those which occur spontaneously during the menstrual cycle were induced by acute elevations in circulating estrogen concentrations in both male and female rhesus monkeys gonadectomized in adulthood. These experiments demonstrate that in primates, in contrast to rodents, exposure of the hypothalamohypophyseal unit to androgens throughout fetal and postnatal development does not prevent the differentiation of the control system that governs cyclic gonadotropin secretion.

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