While a nocturnal decline in serum LH levels in the early follicular phase of the menstrual cycle has been well established, a diurnal variation in serum FSH levels in women has not been demonstrated. We addressed this issue by determining serum LH and FSH levels at 15-min intervals for 24 h in the early follicular phase (EFP; n = 16) and late follicular phase (LFP; n = 10) of the menstrual cycle and in postmenopausal women (PMW; n = 10). Serum estradiol was simultaneously measured at hourly intervals. As expected, EFP, but not LFP and PMW, women had a 15% nocturnal decline (P less than 0.01) in transverse mean LH levels compared to values in the daytime hours. In contrast, nocturnal FSH transverse mean values were significantly lower than daytime values in all groups studied, demonstrating an 18% decline in EFP (P less than 0.001), a 17% decline in LFP (P less than 0.00001), and a 4.3% decline in PMW (P less than 0.01). Cosinor analysis revealed a circadian rhythm for FSH, with acrophases in the afternoon and nadirs at night in all three groups. The circadian amplitudes were 1.43 +/- 0.22, 1.02 +/- 0.16, and 8.42 +/- 1.31 IU/L for EFP, LFP, and PMW, respectively. The EFP nocturnal decline in LH did not conform to a cosine rhythm. A diurnal variation in estradiol was not present in any of the groups of women. These data constitute the first demonstration of a robust circadian rhythm of serum FSH in women. The comparable timing of the acrophase in all groups of subjects and its presence in the postmenopausal years suggest a central, rather than peripheral, feedback mechanism(s) for the circadian rhythmicity. This observation provides strong evidence for a dissociation in the hypothalamic regulation of pituitary LH and FSH secretion in women. The circadian peak and nadir of circulating FSH may prove to be determining for appropriate follicular development.