Serum E1, E2 and E3 concentrations and E2/E1 ratio were measured after vaginal application of conjugated oestrogens, micronized 17 beta-oestradiol and oestriol. 2.4 mg of conjugated oestrogens caused a prompt elevation in the serum E1 concentration; the E2 level changed only slightly. After vaginal application of 2 mg micronized 17 beta-oestradiol the main serum oestrogen is E2 and the conversion of E2 to E1, as in oral administration, does not occur. A significant elevation in the serum E3 concentration was noted 2 h after the vaginal application of 0.5 mg oestriol. The E2/E1 ratio changed little after the application of conjugated oestrogens but increased considerably after the vaginal administration of 2 mg micronized 17 beta-oestradiol.