SUMMARY The metabolism of 17α-ethynyloestradiol was studied in one man and 16 women. After intravenous or oral administration, 22·6–46·9% of the dose was excreted in the urine over a 5-day period. Up to 36% of the urinary radioactivity may be present in the unconjugated form with a further 64% being extractable after enzymic hydrolysis. Sulphate conjugates formed only 11·4% of the urinary radioactivity. Semi-quantitation of these extracts showed ethynyloestradiol itself to be the major metabolite, accounting for 3·2–8·4% of the administered dose in the unconjugated fraction with a further 2·4–16·5% in the enzyme hydrolysed fraction. Up to 16·5% of the dose was therefore excreted unchanged, and metabolites less and more polar than ethynyloestradiol were also found. Excretion via the faeces accounted for up to 30% of the administered dose, with most of the extractable radioactivity being in the unconjugated form. Ethynyloestradiol accounted for up to 29·6% of faecal radioactivity. No significant amounts of radioactivity were detected in sweat collected from one subject 2 or 24 h after administration of the dose. Plasma levels of 3·32% and 1·72% of the administered dose per litre were found 1 and 24 h after administration. Metabolites in plasma were present mainly as sulphate conjugates.