The production of vasoconstrictory thromboxane A2 (TxA2) and vasodilatory prostacyclin ( PG2 ) was studied in women suffering from climacteric vascular instabilities before and during the oestrogen-progestogen therapy. The serum concentrations of TxB2, a metabolite of TxA2, in climacteric patients were similar (170.5 +/- 25.5 ng/ml, mean +/- SE, n = 14) to those in control subjects (196.0 +/- 27.5 ng/ml n = 17) before the start of treatment, but rose to 209.3 +/- 24.5 ng/ml after 3 wk of treatment (P less than 0.01 in comparison with the pre-treatment level), to 227.2 +/- 44.1 ng/ml after 3 mth (P less than 0.05) and to 237.4 +/- 30.3 ng/ml after 6 mth (P less than 0.05). The plasma concentrations of 6-keto-prostaglandin F1a , a stable breakdown product of PG2 , were normal in climacteric (43.5 +/- 7.3 pg/ml as against 46.1 +/- pg/ml) and did not change during replacement therapy. It was concluded, firstly, that climacteric symptoms are not accompanied by changes in TxA2/ PG2 which can be detected in peripheral blood and, secondly, that the increase in the concentration of vasoconstrictory TxA2 induced by oestrogen-progestogen therapy may contribute to the disappearance of climacteric vascular instabilities.