Background. Endothelin is involved in the control of cardiovascular and renal functions and acts as a neuromodulator. Methods. In a prospective study among 15 male patients who underwent coronary artery bypass grafting, we investigated the release pattern and possible stimuli of circulating endothelin. Results. We detected a steep increase in endothelin concentrations after the onset of cardiopulmonary bypass (CPB), and a second minor increase during CPB. The steep increase in endothelin concentrations correlated with the change in arterial pressures at the onset of CPB (r = 0.57; p < 0.03). The slow increase in endothelin concentrations during CPB, however, correlated with mean endotoxin levels during and after CPB (r = 0.60; p < 0.02). Conclusions. The change in arterial pressure at the onset of CPB seems to induce a steep and fast increase in circulating endothelin level, which is probably mediated through the baroreceptors. The slow increase in endothelin level during CPB is associated with increased circulating endotoxin concentration. It may be that either endothelin-mediated vasoconstriction induces endotoxin transmigration from the intestine or endotoxin stimulates endothelin secretion from endothelial cells.