To characterize the differential aging response in gonadotropin secretion that occurs before menopause, we assessed pulsatile LH and serial FSH, estradiol (E2), and progesterone (P) concentrations in aging women across the menstrual cycle. We conducted 96 daytime studies during the follicular, midluteal, and late luteal phases of the same menstrual cycle in 32 volunteers, aged 40-50 yr (n = 16) and 19-39 yr (n = 16). Mean cycle length was shorter in the older women (26 +/- 0.4 vs. 27.6 +/- 0.6 days; P = 0.02), but mean plasma E2 and P values were similar in the two age groups. Mean plasma FSH was higher in the older group on all 3 study days. For LH, an age difference was observed during the late luteal phase, when mean plasma LH and pulse amplitude were higher in women over 40 yr of age (mean LH, 6.4 +/- 0.7 vs. 3.0 +/- 0.5 IU/L (P = 0.002); mean amplitude, 4.0 +/- 0.5 vs. 2.8 +/- 0.2 IU/L (P = 0.03)]. Pulse frequency was higher in the older group, but not different from that in younger women on all study days. When the subjects aged 35-39 yr were analyzed as a third age group (n = 8), age effects for mean LH persisted, and pulse frequency was higher in the group over 40 yr of age vs. women under age 35 yr (n = 8) in both the follicular phase (7.1 +/- 0.4 us. 5.6 +/- 0.8; P = 0.03) and late luteal phase (5.8 +/- 0.7 vs. 4.4 +/- 0.3; P = 0.03). Although highly variable, individual patterns of gonadotropin secretion in the women over age 40 yr included a sustained elevation in the FSH/LH ratio as well as a failure to demonstrate slow frequency, high amplitude LH pulses in the midluteal phase. In conclusion, 1) the age-related increase in FSH concentrations in ovulatory women, although more pronounced, is associated with phase-dependent enhancement of pulsatile LH secretion; 2) the higher LH concentrations are brought about by changes in both pulse frequency and amplitude; and 3) these age effects preempt overt reductions in cyclic E2 or P concentrations.