The effect on serum prolactin, LH and FSH levels of 25 mug ethinyloestradiol administered daily per os during 27 consecutive days was investigated in 5 post-menopausal women aged 52-78. Blood samples were collected before, during and after treatment. The hormones were assayed in serum by radioimmunological methods. Both LH and FSH decreased progressively and significantly from 120 and 115 mIU/ml before treatment to 52 and 51 mIU/ml, respectively after three weeks of oestrogen administration. Two weeks after interruption of treatment, LH (90mIU/ml) and FSH (112 mIU/ml) were significantly higher than during the last week of treatment. Mean prolactin level increased from 127 muU/ml before treatment to 237 muU/ml after 10 days of oestrogen administration (P less than 0.001). This increase was significant after 4 to 8 days and the levels remained about twice as high as the control values for the rest of the treatment period. Two weeks after interruption of treatment, serum prolactin had fallen (136 muU/ml) to the pre-treatment levels. Such results raise the question of possible effects of elevated levels of this hormone during long term oestrogen medication in post-menopausal women on the development of breast cancer.