Blood samples were withdrawn at 10-, 20- or 30-min intervals for 6–12 hr from 13 ovariectomized and 7 intact female rhesus monkeys bearing indwelling cardiac catheters and restrained in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 t½ calculated from 19 such decay curves based on 10-min sampling intervals was 72 min, which is similar to the estimated t½ of exogenous LH in intact females. Moreover, there was a significant positive corre...