The measurement of arterial blood lactate concentration for the purpose of estimating the severity and prognosis of acute perfusion failure is suspect because of theoretical errors due to systemic "lactate washout" immediately following restoration of perfusion. If arterial lactate concentrations continue to increase following resuscitation, the assumption that increasing lactate concentrations indicate progression of anaerobiosis due to perfusion failure would be invalidated. Lactate washout was therefore investigated in a porcine model of cardiac arrest due to electromechanical dissociation. Cardiopulmonary resuscitation was initiated and maintained for intervals of 30 minutes or until spontaneous circulation was restored. In 25 trials on 14 successfully resuscitated animals, the arterial blood lactate concentration decreased within four minutes after resuscitation from cardiac arrest. In 24 animals in whom resuscitation efforts failed, arterial lactate concentrations increased throughout the observation period. Lactate washout occurred during an interval of only 2.6 +/- 0.3 minutes (mean +/- SEM). These results indicate that lactate measurements are not invalidated because of a washout phenomenon under the extreme conditions of cardiac arrest.