In 13 patients undergoing infrarenal aortic bypass operation under neuroleptic anaesthesia, prostaglandins (KH2PGF2 alpha, PGF2 alpha, 6-keto-PGF1 alpha) and thromboxane (TXB2) were measured immediately prior to, 5 min after and 15 min after eventration of the gut. Blood gas analyses were performed at the same points in time. The levels of PGF2 alpha, although slightly elevated, remained stable, as did the levels of TXB2 (more than half the values being below the limit of detection). In 9 patients there was an immediate increase in the level of 6-keto-PGF1 alpha (a stable metabolite of prostacyclin), associated with a significant fall in the arterial oxygen tension. In 4 patients, the levels of 6-keto-PGF1 alpha remained low and no decrease in arterial oxygen tension was observed. KH2PGF2 alpha levels remained within the normal range, indicating that there was no stimulation of general prostaglandin secretion, but an isolated release of prostacyclin. In 7 patients, a mild, moderate or pronounced flush developed which did not, however, correlate to increases in the concentration of 6-keto-PGF1 alpha. These findings indicate that eventration of the gut is followed by prostacyclin liberation in considerable amounts due to the manipulation involved or to the impairment of the intestinal circulation. The concomitant fall in the oxygen tension is caused by pulmonary vasodilation, which increases the perfusion of underventilated parts of the lung.