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Plasma FSH concentrations in rats have been determined by radioimmunoassay under a variety of experimental conditions to see whether any evidence could be obtained of an acute divergence in LH and FSH secretion rates which would support the idea of separate, specific hypothalamic releasing factors for these two hormones. During the normal ovarian cycle and after the administration of progesterone to female rats on the morning of the day of pro-oestrus increased secretion of both LH and FSH began simultaneously but FSH concentrations were later maintained or increased slightly while LHING EARLY PREGNANCY FSH concentrations were higher than at the corresponding stage of the cycle at a time when LH concentrations had been shown to be lower. Progesterone injected at the dioestrous stage of the cycle reduced both LH and FSH concentrations though the effect on LH was more marked. After ovariectomy at any stage of the oestrous cycle or on day 4 of pregnancy there was a rapid and significant increase in plasma FSH concentration which was quite different from the delayed increase in LH concentration observed in these animals. In contrast, the early increase in FSH concentration in male rats after castration was less than the increase in LH concentration. The final FSH concentration in castrated males was only about four times the basal level in contrast to the 10- to 15-fold increase in LH in males and both LH and FSH in females. Anovulatory adult females that had received 1-25 mg of testosterone propionate on day 4 of postnatal life showed the rapid and sustained increase in plasma FSH after ovariectomy that was seen in normal females. None of these results strongly support the idea that separate and specific hypothalamic releasing factors for LH or FSH are secreted in the rat although the differences in the early response to gonadectomy could be explained on this basis.