The effect of infusing the M-cholinergic receptor blocker anisodamine during cardiopulmonary bypass (CPB) on granulocyte-mediated lung injury was evaluated in sixteen dogs undergoing CPB for 120 min with the aorta crossclamped for 90 min. The treated dogs (n = 8) received a total dose of 15 mg/kg anisodamine hydrochloride before and during CPB whereas the controls (n = 8) received saline only. A significantly reduced sequestration of granulocytes in the lungs was found in the treated dogs as indicated by a lower right-to-left atrium granulocyte gradient 15 minutes after pulmonary recirculation during CPB (p less than 0.05) and less granulocyte accumulation in the lung capillaries after CPB, as shown by histological examination (p less than 0.01). Moreover, oxygen free radical release as indicated by plasma malondialdehyde concentrations was significantly lower (p less than 0.05) in the treated dogs than in the controls towards the end of CPB. Finally, a significantly reduced lung-water content was found 30 min after CPB in the treated dogs as compared with the controls (p less than 0.01). These results suggest that anisodamine administration in this experimental model significantly inhibits pulmonary granulocyte sequestration in CPB and the consequent lung injury induced.