The ventilatory response in acute lactic acidosis was assessed in 39 patients. In 18 patients, the acidosis was associated with phenformin ingestion and in 21, with other causes such as shock and sepsis, but not pulmonary edema. Arterial blood CO2tensions and plasma bicarbonate concentrations were compared to those previously found in patients with uncomplicated diabetic ketoacidosis. In most of the lactic acidosis patients, arterial blood CO2fell within the 95% confidence band calculated from the data in the ketoacidotic patients. Only 1 lactic acidotic patient had a triflingly lower CO2tension. Shock was present in 8 of the 9 lactic acidotic patients whose CO2tensions were more than 2 torr above the 95% confidence band.