Abstract. A rapid radioimmunoassay, using a specific antiserum against oestriol (Oe3) was developed for the analyses of unconjugated oestriol in post-menopausal women before and after oral administration of oestriol. One ml plasma was extracted once with 10 ml diethyl ether and assayed without further purification. The plasma levels of oestriol in post-menopausal women appear to be below 5 pg/ml. When 10 ml plasma from a pool was assayed 2.9 pg/ml was found. When 1 ml was assayed from individual patients levels between 10–20 pg was found, which was not different from the plasma blank. A rise in plasma oestriol was found already 15 min after oral administration of 6 mg or 12 mg oestriol (Ovesterin®). On the average there was a plasma oestriol peak after 45 min and then a gradual decrease to low levels within 3 to 4 h. It is concluded that oestriol is rapidly absorbed when administered orally. However, the duration of the plasma oestriol elevation is short. Oestriol is rapidly metabolized. The short elevation probably explains the weak oestrogenic effect exerted by oestriol when administered once a day as receptor affinity for oestriol is low.