Air-purifying respirators impose several loads on breathing. The effects of inspiratory flow resistance, respiratory dead space such as that of a full-face mask, and moderate exercise and maximal exercise were assessed alone and in combination in nine normal volunteers. There were significant effects on respiratory timing variables (e.g., inspiratory time, inspiratory: expiratory time ratio), peak mouth pressure and peak inspiratory flow rate. Only minimal effects on heart rate were noted. These results suggest that prospective workers who must utilize respirators should be observed during exercise as well as at rest and that a respirator dead space and inspiratory resistance should not produce a significant heart rate change but should lead to adaptation of the inspiratory: expiratory time ratio. Workers who show atypical responses should be assessed particularly carefully.