To evaluate the behavior of circulating endothelin and atrial natriuretic peptide (ANP) during coronary artery bypass graft (CABG) surgery, blood samples from patients with coronary artery disease (n = 8) were investigated before, during and after operation. Plasma levels of endothelin and ANP were determined using the radioimmunoassay method. Baseline plasma levels were compared to those of normal volunteers (n = 6). Left ventricular function at rest and as a response to isometric exercise was evaluated using radionuclide ventriculography before and after coronary bypass surgery. The mean endothelin value was found to be within normal limits, however the mean ANP value was slightly higher than control. Patients had significantly improved left ventricular systolic and diastolic function after surgery. The mean endothelin level was higher than initial values immediately after extra-corporeal circulation and returned to initial values in two hours. However, ANP values were increased and remained higher than initial values. Baseline endothelin values were negatively correlated with systolic function parameters, whereas endothelin and heart rate had a positive correlation before extra-corporeal circulation. Coronary artery bypass graft surgery may cause an increase in the circulating endothelin level either due to endothelial injury or due to myocardial ischemia and hypothermia. Following surgery, increased endothelin levels returned to normal values immediately.