A relative pyridoxine deficiency was found in all of 12 women using conjugated oestrogens unopposed by progestagsns. This was due to disturbed tryptophan metabolism, expressed in increased xanthurenic acid (XA) excretion (greater than or equal to 60 mumol/8 h) during 8 h following oral administration of 2 g L-tryptophan. The intake of synthetic oestrogens such as ethinyl oestradiol has already been found to lead to a disturbance of tryptophan metabolism and to a deficiency of vitamin B6. Now we have evidence that this is the case not only in women taking oestrogens in oral contraceptives but also as replacement therapy during the postmenopause. This disturbance is clear after 1 yr of oestrogen treatment. Xanthurenic acid excretion was only slightly increased in 3 women who used progestagens in high dosages at the same time. The biochemical changes induced could easily be corrected by administration of vitamin B6. Our cyclic treatment regimen now consists of 25 days of oestrogens per month. In the remaining days a 250 mg tablet per day of vitamin B6 is prescribed.