Luteinizing hormone (LH), follicle-stimulating hormone (FSH), oestradiol and progesterone concentrations in plasma were obtained daily throughout the menstrual cycles of 94 regularly cycling women, aged between 24 and 50 years. Although mean LH concentrations changed little with advancing age, mean FSH concentrations were significantly (P less than 0.001) elevated from the age of 39 years. FSH concentrations in the oldest women studied (48-50 years) were approximately 3-fold greater than in the younger controls (women aged 23-35 years). LH concentrations rose slightly (P less than 0.05) during the last 5 years only. The increase in FSH concentration was not, however, uniform across the cycle, but was confined predominantly to the mid-follicular and post-ovulatory phases (i.e. those times in the normal menstrual cycle when circulating inhibin concentrations appear to be minimal). Despite the clear increases in FSH concentration, there was little alteration in the mean steroid profiles which remained within the normal fertile range throughout the last decade of reproductive life. The only exception to this was a small, transient, but significant (P less than 0.05) decrease in preovulatory oestradiol concentration between the ages of 36 and 38 years, which was followed by a transient increase (P less than 0.01) in oestradiol concentration between 39 and 44 years. However, no corresponding significant changes in mean progesterone concentrations were observed.