Blood samples were collected from adult Soay rams at frequent intervals during the regressed, developing and active phases of the sexual cycle, or after i.v. injections of synthetic LH-RH or standard preparations of ovine FSH and LH, and were assayed for FSH, LH and testosterone. The highest FSH and LH levels occurred during the developing stage of the cycle, and the highest testosterone levels during the active stage. At each phase there were conspicuous short-term fluctuations in the concentrations of all three hormones, and episodic peaks of plasma LH were associated with increases in both FSH and testosterone. Synthetic LH-RH (5 microgram) induced a rise in the plasma values of FSH and LH at all stages and the magnitude and duration of the response could be predicted from a knowledge of the endogenous short-term fluctuations. When 50--250 ng LH-RH were given i.v. plasma LH increased in a way similar to that occurring spontaneously. The ratio of FSH:LH released after LH-RH changed with the stage of the sexual cycle, but the clearance rate of the two hormones was not affected. These findings are consistent with the control of FSH and LH by a single releasing hormone which is secreted in pulses. The different temporal patterns in the circulating FSH and LH result from differences in secretion and clearance.