Physiologic fluctuation of follicle-stimulating hormone (FSH), luteinizing hormone (LH), human-growth hormone (HGH), and progesterone concentrations in serum samples of women with ovulatory menstrual cycles were determined by radioimmunoassay and protein-binding assay. During the follicular phase of the cycle, the tonic secretion of FSH showed an initial rise followed by a progressive fall toward the midcycle. In contrast, LH secretion showed a steady increase. A clear elevation of progesterone concentration was observed 48 hours after the onset of a simultaneous surge of LH and FSH at midcycle. Progesterone levels rose rapidly reaching maximal concentration (mean 12.2 mμg/ml) five days after LH surge and remained elevated for five to six days. Thereafter, progesterone levels fell rapidly to an undetectable level on the day after onset of menstruation. This was accompanied by a significant elevation of the FSH and LH levels from their low concentration observed during the late luteal phase. Human-growth hormone levels observed while the patient was in a fasting, ambulant state showed a progressive rise reaching a peak just prior to LH surge and there was a random fluctuation during luteal phase of the cycle.