To define the time course of the metabolic acidosis that follows a single grand-mal seizure, we obtained serial blood samples from eight consecutive patients. Immediately after a seizure, the mean (+/- S.E.M.) venous lactate concentration was 12.7 +/- 1.0 meq per liter, the mean carbon dioxide content 17.1 +/- 1.1 mmol per liter, and the mean arterial pH 7.14 +/- 0.06. Sixty minutes later their values were 6.6 +/- 0.7 meq per liter (P less than 0.005), 23.6 +/- 1.1 mmol per liter (P less than 0.005) and 7.38 +/- 0.04 (P less than 0.005) respectively. The spontaneous resolution of the acidosis was due, in large part, to the metabolism of lactate and to the concomitant removal of hydrogen ion. There was no change in the serum potassium concentration, despite the development of a severe systemic acidemia and the subsequent return to normal of the pH. We suggest that the patient with seizures may serve as a unique model of lactic acidosis.