The effects of estriol succinate (Synapause, 2 mg daily) on the renin-aldosterone system and blood pressure (RR) were studied in 14 postmenopausal women after bilateral oophorectomy. Plasma renin activity (PRA) and daily urinary aldosterone excretion (dU-Ald) were determined 1 mth after the operation and before estrogen treatment, at the end of 2 mth therapy, and, for the third time, 2 mth after the termination of treatment with the drug. No changes in PRA, dU-Ald or RR were found in normotensive women, or in 3 women with hypertension in this group. Another group of 11 postmenopausal women was investigated after long-term estriol succinate therapy, which had lasted for 5-8 yr after oophorectomy. PRA, dU-Ald and RR were measured during treatment and 2 mth after terminating the therapy. No changes were found either in hormone differences in the mean levels of PRA or dU-Ald. The results suggest that estriol succinate is devoid of general harmful effects on the renin-aldosterone system during postmenopausal therapy for climacteric symptoms.