The effects on plasma 17-β-oestradiol (E 2), oestrone (E 1), 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 E 2 and E 1 increased progressively and significantly (p0.05). LH and FSH decreased progressively and significantly (pp>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 is maintained and that there is no increase of prolactin. In this way, 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