Seven postmenopausal women have been treated daily with 3 mg oestradiol percutaneously applied upon the skin. Blood samples were drawn at 8-h intervals during a 4-day period and on days 5, 7 and 9 from the beginning of the treatment. Plasma Plasma oestradiol (E2), oestrone (E1), follicle stimulating hormone (FSH) and luteinizing hormone (LH) were determined by radioimmunoassay on these samples. The plasma E2 level was significantly increased in the 12th hour (73 +/- 17 pg/ml) but the maximal plasma concentration was obtained only at the third day of treatment (110 +/- 24 pg/ml). Thereafter the mean plasma concentration was more stable. Increments in E1 was smaller and the plasma E2/E1 ratio was 1.51. Plasma FSH and LH di not change significantly during the course of the treatment. Thus the percutaneous administration of E2 appears to be an effective and safe method of delivering E2 into the circulation, and mimicking the physiologic condition. The advantages of this method are discussed.