Survival after out-of-hospital cardiac arrest is related to the time from cardiovascular collapse to the initiation of cardiopulmonary resuscitation (CPR}, or downtime. Blood lactate levels have been shown to be an objective and reliable predictor of downtime in dogs subjected to cardiopulmonary arrest and openchest resuscitation. The purpose of this study was to determine the value of blood lactate levels in predicting downtime in dogs subjected to cardiopulmonary arrest and closed-chest CPR. Ascending aortic and Swan Ganz catheters were placed in 14 adult mongrel dogs. After baseline arterial lactic acid samples were obtained, ventricular fibrillation was electrically induced and the animal remained in ventricular fibrillation without ventilation for 5 minutes. At the end of 5 minutes of cardiac arrest, a continuous infusion of epinephrine was begun, and standard external CPR was initiated using a mechanical chest compressor and ventilator. Arterial lactic acid samples were obtained every 5 minutes during 30 minutes of CPR, after which defibrillation and resuscitation were attempted. Successful resuscitation was defined as a persistent rhythm and a systolic blood pressure greater than or equal to 80 mm Hg within 10 minutes of initial countershock. Arterial lactic acid samples were obtained at timed intervals after resuscitation. Mean arterial lactic acid levels increased significantly with each sampling interval during 30 minutes of standard external CPR (overall P < .05). In 8 dogs successfully resuscitated, there was no significant difference in mean arterial lactic acid levels after the return of spontaneous circulation. Resuscitation in dogs utilizing open-chest CPR results in insignificant changes in mean lactate levels once cardiac massage is initiated. In contrast, closed-chest CPR in dogs does not provide adequate tissue oxygenation and/or perfusion to halt anaerobic glycolysis and further lactic acid accumulation. Lactate levels are not predictive of downtime in dogs subjected to cardiorespiratory arrest and closed-chest CPR.