Presently, there is no reliable noninvasive method of assessing the adequacy of cardiopulmonary resuscitation (CPR). Studies of animals have shown that during prolonged arrest the coronary perfusion pressure (CPP) is correlated with successful resuscitation. During previous studies it appeared that expired PCO2 correlated with CPP. To investigate this relationship, eight mongrel dogs (mean weight, 22.7 +/- 5.8 kg) were anesthetized with pentobarbital. Catheters were placed in the thoracic aorta and right atrium of each dog. Each animal was electrically fibrillated, and CPR was started using mechanical resuscitator. The PCO2 was determined at end expiration using a Hewlett Packard 47210A Capnometer with the electrode attached to the endotracheal tube. After 10, 15, 20, or 25 minutes of ventricular fibrillation and closed-chest massage, a thoracotomy was performed, and internal massage was begun. Coronary perfusion pressure was calculated at least each minute and correlated with the PCO2 values. A correlation coefficient of 0.78 was calculated based on 368 data points for eight dogs (P less than 0.01). The results of this study indicate that expired PCO2 is positively correlated with CPP in the canine model of CPR. Inasmuch as CPP correlates with survival in prolonged CPR, the noninvasive measurement of PCO2 may be a useful method of assessing the adequacy of CPR.