A method for the estimation of unconjugated aldosterone in the plasma of pregnant women has been developed. The method is suited to routine analysis of a large number of samples, both from pregnant and non-pregnant subjects. An ethyl acetate extract of plasma is oxidized with sodium periodate to destroy interfering lipids and corticosteroids. Aldosterone is quantitatively converted to aldosteroid-gamma-lactone which is estimated by radioimmunoassay. The method has high specificity and an overall recovery of 80 +/- 5%. The method has been used to establish circulating levels of aldosterone during pregnancy and the menstrual cycle. In early pregnancy, levels of aldosterone were 2-3 times higher than in the follicular phase of the menstrual cycle and a further significant rise occurred at the beginning of the third trimester. No correlation could be detected between plasma aldoserone and plasma progesterone, nor between plasma aldosterone and the birth weight of the baby. Plasma aldosterone levels in women who subsequently developed hypertension were no different from those in women who remained normal throughout pregnancy.