To determine differences in regional blood flow during CPR versus normal cardiac function, the authors measured regional blood flow to several organs in 19 pentobarbital-anesthetized dogs (6--12 kg). Regional blood flow was measured during sinus rhythm in five dogs and during electrically induced ventricular fibrillation with CPR in the other 14 dogs. Regional blood flow and cardiac output were measured using radioactively labeled polystyrene microspheres of 15 +/- 3 mu diameter, injected into the left ventricle. Adequacy of microsphere mixing at low cardiac outputs was verified by comparing flow rates to paired organs. Cardiac output was 175 ml/kg . min during sinus rhythm versus 47 ml/kg . min during CPR. Flow to all organs sampled was less during CPR, but the relative decrease varied widely. The ratios of regional blood flow during CPR to regional blood flow during sinus rhythm were 90% for brain, 35% for heart, 15% for kidneys, 17% for adrenal glands, 14% for pancreas, 3% for spleen, and 33% for small intestine. These results provide baseline values for regional blood flow during CPR which can be used to evaluate alternative CPR techniques and/or drugs which may improve perfusion of vital organs during CPR.