In spite of animal data showing an effect of melatonin in the regulation of the hypothalamus-pituitary-adrenal (HPA) axis, no effect of melatonin on cortisol has been evidenced in young men. Gender and aging are believed to influence the regulation of the HPA axis, and may thus modulate the melatonin effect on cortisol. In this study we investigated whether an effect of melatonin on cortisol can be observed in women of different age. Six young women in early follicular phase (22-32 years; EFW) and eight aged women in postmenopause (54-62 years; PMW) were studied. At 08.00 h on two consecutive days each woman received, randomly and in double-blind fashion, a pill of placebo or melatonin (100 mg). Serum levels of melatonin and cortisol were evaluated at 20-min intervals for 48 h. In comparison to EFW, PMW showed an earlier onset of nocturnal melatonin (p < 0.05) and cortisol rise (p < 0.01) and higher cortisol levels at lunch (p < 0.05) and early evening (p < 0.01). Melatonin administration did not modify serum cortisol levels in EFW but elicited a marked increase of daytime cortisol levels in PMW (p < 0.02). The present data reveal that in aged PMW the cortisol levels are enhanced at selected circadian times and are stimulated by melatonin.