1. The object of this study was to investigate clinical conditions in which increased production of prostacyclin (PGI 2 ) has been reported. 6-Oxo-prostaglandin F 1α (6-oxo-PGF 1α ) is the stable hydrolysis product of PGI 2 and was measured in plasma from patients undergoing hepatic or cardiac surgery and in unoperated patients with vascular and hepatic disease, using gas chromatography/mass spectrometry. 2. Blood obtained simultaneously from portal and peripheral veins, during emergency surgery for bleeding oesophageal varices in six patients with cirrhosis of the liver, contained very high concentrations of 6-oxo-PGF 1α (range 99–11485 pg/ml of plasma). 6-Oxo-PGF 1α was higher in portal than in peripheral blood in five out of six patients. 3. Six unoperated patients with cirrhosis and oesophageal varices which were not bleeding all had normal peripheral plasma concentrations of 6-oxo-PGF 1α < 2 pg/ml (normal up to 5 pg/ml). 4. Seventeen patients with severe vascular disease had normal basal plasma 6-oxo-PGF 1α concentrations (< 2 pg/ml). 5. Eighteen subjects with atheromatous coronary artery disease underwent aorta–coronary artery grafting, and plasma concentrations of 6-oxo-PGF, α were markedly elevated during surgery (range 55–1207 pg/ml). 6. We conclude that surgery stimulates PGI 2 production substantially, and argue that the function of PGI 2 may be to limit intravascular extension of thrombus from sites of haemostasis. Inappropriate PGI 2 synthesis may contribute to the massive haemorrhage characteristic of oesophageal variceal bleeding.