Several groups of investigators have previously reported that small amounts of hCG are present in blood and urine of nonpregnant eugonadal women and men. We have developed highly sensitive and specific, two-monoclonal antibody, sandwich-type assays which can quantify both hCG and LH in sera from postmenopausal women, women at all phases of the menstrual cycle, and men. Using these assays we have also reported that hCG is secreted in a pulsatile fashion in postmenopausal women, is stimulated by GnRH in both men and postmenopausal women, and is suppressed by a GnRH agonist in castrate men. Employing these same sandwich assays, we report herein that hCG is secreted in a pulsatile manner during the follicular and luteal phases of the normal menstrual cycle. During the follicular phase 0.68 +/- 09 (+/- SEM) pulses of hCG occurred each hour, while 0.62 +/- 0.08 pulses of LH occurred. Pulse durations during the follicular phase for hCG and LH, respectively, were 38.3 +/- 4.4 and 62.5 +/- 11.1 min (P less than or equal to 0.05). During the luteal phase there were 0.42 +/- 0.16 pulses/h of hCG and 0.38 +/- 0.08 pulses of LH (P greater than 0.05). Pulse durations were 22.3 +/- 7.5 and 126.4 +/- 19.0 min for hCG and LH, respectively (P less than 0.01). The t1/2 of hCG disappearance was 37.2 +/- 3.8 min during the follicular phase and 22.9 +/- 4.6 min during the luteal phase. The t1/2 values of LH were 82.9 +/- 5.7 and 67.5 +/- 5.12 min during follicular and luteal phases, respectively. The t1/2 of LH was greater than the t1/2 of hCG (P less than 0.01). We conclude that small amounts of hCG are secreted in a pulsatile manner during follicular and luteal phases of the human menstrual cycle.