Six ovariectomized women with the uterus left in situ were given a daily oral dose of 0.5 mg of oestradiol decaneoate in oil for 14 days. After a single dose of oestradiol decanoate mean plasma oestradiol rose from 26.8 pg/ml to 66.8 pg/ml within 30 min. There was a further rise to 100-110 pg/ml (P less than 0.001) remaining at the same level over a 24 h period. On repeated daily administration of oestradiol decanoate this rise continued reaching the mean value of 187.1 pg/ml (P less than 0.001) at 2 weeks, while plasma oestrone remained at the same level (147.1 pg/ml) as it was at 24 h. The ratio of plasma oestrone:oestradiol was less than 1 at 2 weeks. Plasma FSH and LH fell progressively during the medication. Changes of the Maturation Index, cervical mucus Ferning and Spinnbarkeit demonstrated the strong 'peripheral' activity of this dose of oestradiol decanoate. All endometrial specimens showed late proliferative changes at 2 weeks. It was concluded that the favourable properties of oestradiol decanoate offer new possibilities for a more physiological oestrogen supplementation therapy.