Plasma immunoreactive inhibin levels have been measured in a series of normal conception cycles (group I; n = 7), and the data compared to inhibin concentrations in normal menstrual cycles (group II; n = 8), in women with luteal phase defects (group III; n = 7), and in women in the perimenopausal period (group IV; n = 6). Daily plasma levels of LH, FSH, progesterone, estradiol, and inhibin were determined in each subject, and daily mean profiles for each hormone in each subject group were calculated and expressed as geometric means with 68% confidence limits. During the follicular and early luteal phases, inhibin concentrations in the normal nonpregnant group (group II) were significantly higher than those in the conception cycles of group I, but after implantation in the conception cycles, inhibin concentrations increased to levels in excess of those seen at any time in nonconception cycles (716-1352 U/L; P less than 0.02). The postimplantation rise in inhibin did not initially appear to follow the same pattern as progesterone. While progesterone concentrations rose within 24 h of the first detectable increase in hCG, inhibin levels did not increase until 3 days later, although after this point concentrations increased serially and in parallel with progesterone. LH and FSH concentrations were markedly suppressed after implantation. Follicular and early luteal inhibin concentrations in cycles with luteal phase defects were also higher than those in conception cycles, although this difference was only significant in the midfollicular phase. Follicular phase inhibin concentrations in cycles from older women (group IV) were lower than those in groups II and III, but were not distinguishable from those in the conception cycles. Estradiol concentrations in the same subjects were significantly lower during the early follicular phase, while follicular and luteal FSH concentrations were significantly higher than those during conception cycles. Finally, examination of the relationship between inhibin, FSH, and estradiol around menstruation in the older women revealed a far closer temporal association between FSH and estradiol than between FSH and inhibin. In conclusion, inhibin concentrations rise and fall throughout the human menstrual cycle in a manner that is similar to but at specific times significantly different from that of either of the ovarian steroids estradiol and progesterone. It is considered to be a peptide of granulosa cell origin and may be an indicator of the size of the follicular pool during the early stage of the cycle. However, although there is some degree of inverse correlation between profiles of inhibin and profiles of FSH, this relationship is not particularly clear.(ABSTRACT TRUNCATED AT 400 WORDS)