Nine patients (seven men and two women) were studied while undergoing coronary artery bypass on cardiopulmonary bypass. Selective bronchial flow samples were obtained and analyzed for prostaglandin E2 levels, and the hemodynamic effects of this vasodilator were studied. Bronchial flow collection and measurements were performed during hypothermic cardioplegic arrest while the peripheral anastomosis was being completed, as described previously. This collected sample was reinfused abruptly to the pump circuit, but samples were also analyzed for specific radioimmunoassay antiserum for prostaglandin E2 levels. Urine levels were obtained both with and without indomethacin block. All nine patients were studied for the hemodynamic effects of rapidly reinfused bronchial flow. A 34% +/- 8.8% mean drop of blood pressure and peripheral vascular resistance were recorded (p less than 0.002). Three study groups were established: In Group I, serum prostaglandin assay was performed on six patients. Prostaglandin E2 levels showed an average of increase of 159% compared to the baseline (p less than 0.02). In Group II, urine prostaglandin was measured. Nine assays were performed on six patients. Analysis of the total urine production while on cardiopulmonary bypass showed an average increase in prostaglandin E2 of 300% (p less than 0.02). In Group III, combined serum and urine prostaglandin levels were measured in three patients before and after indomethacin block (50 mg four times a day for 48 hours before the operation). All of these patients demonstrated the same hypotensive phenomena with reinfusion of the bronchial flow. The urinary prostaglandin E2 output and serum prostaglandin E2 levels in bronchial flow were elevated. Overall, this group manifested a 50% reduction in prostaglandin E2 production over baseline values with indomethacin block (p less than 0.02) and a 300% increase in production during cardiopulmonary bypass (p less than 0.02). Our data suggest that significant amounts of prostaglandin E2 are released in the lung during the stress of cardiac operations. Rapid reinfusion of bronchial flow is responsible for hypotension during cardiopulmonary bypass. Pretreatment with indomethacin will not completely block prostaglandin E2 release in the lung during stress.