To characterize the spectrum of pulsatile gonadotropin secretion during the course of the normal menstrual cycle, we studied normal women during 51 ovulatory cycles. Plasma gonadotropin concentrations were measured at 10-min intervals for 20-24 h during the early, mid-, and late follicular phases and the early, mid-, and late luteal phases. LH data series were analyzed using 2 different computer-assisted algorithms for pulse detection. The LH interpulse interval decreased during the follicular phase (FP) from 94 +/- 4 (+/- SEM) min in the early FP (EFP) to 71 +/- 4 min by the late FP (LFP; P less than 0.001). The estimation of LH pulse frequency in the EFP was significantly affected by slowing of episodic LH secretion during sleep. In the luteal phase (LP), the LH interpulse interval progressively increased from 103 +/- 8 min in the early LP (ELP) to 216 +/- 39 min by the late LP (LLP; P less than 0.001). Sleep-associated slowing of episodic LH secretion also occurred in the ELP. The mean LH pulse amplitude in the EFP (6.5 +/- 0.4 mIU/ml) decreased significantly by the midfollicular phase (MFP; 5.1 +/- 0.8 mIU/ml; P less than 0.05) and increased once again by the LFP (7.2 +/- 1.2 mIU/ml). LH pulse amplitude was highest in the ELP (14.9 +/- 1.7 mIU/ml), decreased by the midluteal phase (MLP) to 12.2 +/- 2.0 mIU/ml, and declined further by the LLP to 7.6 +/- 1.1 mIU/ml (P less than 0.001 vs. ELP). FSH secretion was significantly (P less than 0.05) correlated with LH secretion at time lags of 0-10 min in 82% of the studies. These results indicate the following. 1) In the EFP and ELP, the frequency of gonadotropin pulsations is reduced at night in association with sleep. 2) The frequency of LH secretion increases from the EFP to MFP and LFP. 3) LH pulse amplitude decreases in the MFP, suggesting enhanced negative feedback of estrogen on the hypothalamic-pituitary axis and/or a decrease in GnRH secretion at this stage. 4) A progressive reduction of LH pulse frequency and amplitude occurs during the LP which is correlated with the duration of exposure of the hypothalamic-pituitary axis to progesterone. 5) A close relationship exists between secretion of LH and FSH, suggesting a common stimulatory factor for both gonadotropins.