The contribution of the lung to the clearance of exogenous dopamine after cardiopulmonary bypass (CPB) was analyzed quantitatively in humans and compared with the contribution of the lung before CPB. The pulmonary and arterial plasma concentration of dopamine and the pulmonary plasma flow were measured simultaneously during infusion of dopamine. Contribution of the pulmonary circulation was defined as the ratio between clearance through the pulmonary circulation and the total plasma clearance of dopamine. The calculated contribution values after CPB were 12.0, 10.7, 11.4, 16.2, and 16.7% at the doses of 3.0, 4.0, 5.0, 6.0, and 7.0 micrograms.kg-1.min-1, respectively. Those values before CPB were 15.6% and 17.4% at the doses of 1.0 and 2.0 micrograms.kg-1.min-1, respectively. The comparison of the values before and after CPB did not achieve statistical significance. Furthermore, there were no significant correlations between the pulmonary clearance after CPB and mean pulmonary arterial pressure, pulmonary vascular resistance, or CPB time. The results suggest that the pulmonary clearance mechanism for dopamine after CPB is maintained as effectively as that before CPB and is not influenced by pulmonary hypertension or CPB time.