We have earlier reported on a prolonged effect of orally administered oestriol caused by its enterohepatic recycling after reabsorption from the intestine. The aim of the present study was to test if oral administration of activated charcoal could inhibit the enterohepatic recirculation of orally given oestriol. Plasma concentrations of unconjugated oestriol were measured using a specific radioimmunoassay (RIA). Twelve mg oestriol administered orally to postmenopausal women resulted in elevated plasma oestriol levels for more than 24 h. Plasma oestriol fluctuations in relation to meals were seen. When activated charcoal was given 3 h after oestriol administration, the plasma oestriol concentration declined without further fluctuations and returned to the pretreatment value within 6 h. Our data indicate that oestriol given orally undergoes enterohepatic recirculation after reabsorption from the intestine since administration of charcoal, which binds steroids, resulted in a rapidly declining oestriol level. It is concluded that the prolonged oestriol elevation, which is normally seen, is caused by enterohepatic recycling.