The purpose of this study was to determine the role of lactic acidosis in the ventilatory response to heavy exercise above anaerobic threshold. Seven subjects ingested either NaHCO3 or CaCO3 at a dose of 300 mg/kg body weight and ran on a motor-driven treadmill at a work load corresponding to 90% of VO2max and above anaerobic threshold for a period of 5 min while minute ventilation and PETCO2 were recorded breath by breath. A total of 10 runs, 5 with CaCO3 and 5 with NaHCO3 in a randomized and blind order, were done in each subject. Statistical analyses of the effects of the chemicals on minute ventilation during the 15 s between min 4.75 and 5 of exercise showed that the differences in ventilation did not reach statistical significance (p greater than 0.05) in 5 of the 7 subjects. Venous pH measurements at the end of exercise revealed a significant increase with NaHCO3 (p much less than 0.05). It is concluded that lactic acidosis is not an essential determinant of ventilatory response to heavy exercise above anaerobic threshold in the majority of the subjects.