Catabolism of methionine is supposed to proceed via two known pathways: transsulphuration and transamination. In 10 premenopausal women and 13 young men we measured methionine, the transsulphuration metabolite homocysteine, and the transamination metabolites 4-methylthio-2-oxo-butryate and methanethiol mixed disulphides in the fasting state as well as after oral administration of 0.1 g L-methionine kg-1 body weight. Both in the fasting state and after methionine loading the concentrations of homocysteine in serum were significantly lower in premenopausal women than in young men. Since there is evidence that even a moderate homocysteinaemia may be a risk factor in the development of vascular disease, the low homocysteine levels could be an additional factor contributing to the lower incidence of vascular disease in premenopausal women. After oral methionine these women showed significantly higher concentrations both in serum and urine of the transamination metabolites than the group of men. This higher methionine transamination in premenopausal women may contribute to keeping the homocysteine levels low and may therefore have an impact on the protection of these women against vascular disease.