The physiological role of inhibin and its relation to other sex hormones (estradiol, progesterone, follicle stimulating hormone (FSH) and luteinizing hormone (LH)) has been investigated during gonadotropin-stimulated cycles of 38 in vitro fertilization-embryo transfer/gamete intrafallopian transfer (IVF-ET/GIFT) patients. Human menopausal gonadotropin (hMG) was given from day 3 of the cycle until 1 day before ovulation induction with human chorionic gonadotropin (hCG). Blood samples were taken twice daily and hormone measurements performed by radioimmunoassay or enzyme immunoassay. Patients were divided into two groups: Group A comprised patients < 35 years of age (n = 20) and Group B included patients > or = 35 years of age (n = 18). The pregnancy rate was significantly higher in Group A. During the follicular phase, serum inhibin level rose gradually in both groups but the values were higher in Group A (significantly between days -2 and 0). During the early luteal phase serum inhibin concentrations were similar in both groups. Estradiol pattern did not differ in the two groups. Estradiol pattern did not differ in the two groups. Whilst serum estradiol level did not increase significantly after day 0, serum inhibin concentration reached its peak value 1 day later, on day +1. Serum progesterone was higher in Group A between days +1 and +4 (significantly on days +1, +3 and +4). Serum FSH increased slowly in both groups and did not correlate with serum inhibin concentration. Basal LH concentrations were similar between days -6 and -2 in both groups. Around the time of ovulation induction (day -1, 0 and +1) serum LH was lower in Group A (significantly on day 0).(ABSTRACT TRUNCATED AT 250 WORDS)