To test effects of respirator masks on maximum oxygen uptake, ventilation threshold, and lactate threshold, 14 subjects underwent incremental bicycle exercise with and without masks. There was a statistically significant difference in final oxygen consumption between the two conditions, but other results suggest that the physiological measure of maximum oxygen uptake was not reached at termination for the full-mask condition. Lactate and ventilation thresholds were not affected by mask condition. Hypoventilation while wearing masks caused higher amounts of blood lactate accumulation. The resulting higher mask CO₂ levels and lower O₂ levels, normally attributed to mask dead volume, may instead be due to mask resistance.