OBJECTIVE In women over the age of 45 years with continuing regular menstrual cycles, follicular phase FSH levels rise without an accompanying change in LH. We determined the effect of increasing age in women with regular cycles on the serum levels of FSH, LH, immunoreactive inhibin, progesterone and oestradiol. METHODS Single blood samples were taken during the early follicular phase (days 4-7) and again in the midluteal phase (3-12 days before the next menses) of the menstrual cycle. METHODS Regularly cycling women aged 21-49 years participated in the study (and were grouped into four groups: 20-29, 30-39, 40-44 and 45-49 years in the follicular phase and three groups: 20-29, 30-39 and 40-49 years in the luteal phase. METHODS Serum levels of FSH, LH, oestradiol, progesterone and immunoreactive inhibin were measured from the blood samples obtained. RESULTS Follicular phase Mean follicular phase levels of immunoreactive inhibin were significantly lower in the 45-49 year age group (P less than 0.05) than in the younger age groups (128 U/l in the 45-49 year age group vs 239, 235 and 207 U/l in the 20-29, 30-39, 40-44 year age groups respectively), while mean FSH levels were significantly higher in the 45-49 year age group (P less than 0.05, 13.0 IU/l in the 45-49, 4.9, 5.5 and 5.2 IU/l in the 20-29, 30-39 and 40-44 year age groups respectively). Mean oestradiol levels in the 45-49 year age group were significantly lower only when compared to age group 30-39 years (P less than 0.05, 130 vs 210 pmol/l). There was no significant difference in oestradiol levels between the 45-49 year age group and the 20-29 and 40-44 year age groups. LH levels did not differ significantly across age groups. There was also a significant negative correlation between serum immunoreactive inhibin and FSH (r = -0.45, P less than 0.05) and between oestradiol and FSH (r = -0.35, P less than 0.05). There was a significant negative relationship between immunoreactive inhibin and age (r = -0.46, P less than 0.05). For every 10-year increase in age, average immunoreactive inhibin decreased by an estimated 49.3 U/l. As age increased, average FSH levels exhibited a two-phase linear increase with the change-point estimated at 42.97 (1.42) (estimate (SE)) years. Prior to 42.97 years, FSH barely changed; after 42.97 years there was a significant (P less than 0.05) increase in FSH as age increased. Oestradiol levels did not change significantly until an estimated 37.9 years of age, but then decreased significantly (P less than 0.05) with increasing age. Luteal phase Levels of FSH, LH, serum immunoreactive inhibin, oestradiol and progesterone fell slowly with increasing age. There was a significant correlation between serum immunoreactive inhibin with progesterone (r = 0.41, P less than 0.05) but there was no correlation between serum immunoreactive inhibin LH or FSH. CONCLUSIONS The results are consistent with a role for serum immunoreactive inhibin, in addition to oestradiol, in the regulation of FSH during the follicular phase of the menstrual cycle as a function of increasing age. This is postulated to reflect diminished folliculogenesis as age progresses with the known decline in the numbers of primordial follicles in the ovary as the menopause approaches.