The effect of high doses of ethinyl estradiol (0.4 mg/day) on renin substrate concentration, renin activity and aldosterone concentration in plasma was studied in eight ovariectomized women. Plasma renin substrate (angiotensinogen) increased already within 24--48 h, reaching a maximum on the third to fifth day after starting estrogen treatment. Thereafter, renin substrate concentration remained relatively constant in a range which was fourfold above the baseline levels. The increase of plasma renin activity was less pronounced and showed considerable between-patient variability; this increase was statistically significant only after 48 hours of estrogen intake. A rise in plasma aldosterone concentration was observed in two of four subjects examined. In one patient treated with 5 mg estradiol benzoate intramuscularly, plasma renin activity increased without any measurable change in renin substrate concentration. Only in one subject treated with ethinyl estradiol did plasma renin activity increase before plasma renin substrate concentration; the results presented do not preclude factors other than the stimulation of renin substrate synthesis in the liver from contributing to the activation of the renin-aldosterone axis during treatment with ethinyl estradiol.