The effects on plasma 17-beta-oestradiol (E2), oestrone (E1), LH, FSH and prolactin (PRL) levels of 1.5 mg conjugated oestrogen, administered daily per os for 20 consecutive days, was investigated in six postmenopausal women aged 60-68. Both E2 and E1 increased progressively and significantly (p less than 0.001) from 18 and 28 pg/ml to 32 and 108 pg/ml, respectively, at the end of treatment; five days after the last dose both E2 and E1 had fallen to pretreatment levels (p greater than 0.05). LH and FSH decreased progressively and significantly (p less than 0.001) from 114 and 105 mlU/ml (before therapy) to 43 and 36 mIU/ml, respectively, after oestrogen administration. One week after interruption of treatment, both LH and FSH were significantly higher (p less than 0.001) than that obtained at the end of therapy. No significant variation (0.05 greater than p greater than 0.02) was observed for plasma PRL during and after oestrogen administration. Such results indicate that in postmenopausal women the specific enzymatic mechanism of oestrogen interconversion are maintained and that there is no increase of prolactin. IN this was, the possible effects on the development of breast cancer by elevated levels of this hormone, usually observed during long-term oestrogen therapy, would be avoided.