A comparison was made of respiratory function in submersed divers breathing with either a mouthpiece or a full face mask while exposed to varying depths (15 and 190 fsw), exercise loads (0-175 W), and static lung loads (0 and -20 cmH2O). The two types of breathing equipment were designed to be identical in terms of functional dead space volume and resistance to gas flow. When compared with data from experiments utilizing a full face mask, use of a mouthpiece caused a modest fall in expired minute volume at both depths. The majority of this decline may have been the consequence of a decrease in dead space ventilation brought about by the elimination of simultaneous nose breathing and mouth breathing. Alveolar ventilation and PETCO2 were not significantly influenced by the use of a mouthpiece, regardless of depth, workload, or static lung load. With both types of breathing gear episodes of dyspnea were infrequent during experiments with a static lung load of 0 cmH2O. Therefore, if a neutral static lung load is maintained, the type of breathing gear used does not seem to be of consequence as far as dyspnea is concerned.