Our previous work showed that breathing low density gases during exercise with the self-contained breathing apparatus (SCBA) improves maximal ventilation (V(E)) and maximal oxygen consumption (VO(2)max). This suggests that the SCBA limits exercise by adding a resistive load to breathing. In this study we compared VO(2)max with and without the various components comprising the SCBA to determine their impact on VO(2)max. Twelve males performed 4 randomly ordered incremental exercise tests to exhaustion on a treadmill: (1) low-resistance breathing valve only (CON); (2) full SCBA (SCBA); (3) SCBA regulator only (REG); and (4) carrying the cylinder and harness assembly but breathing through a low-resistance breathing valve (PACK). Compared to CON, VO(2)max was reduced to a similar extent in the SCBA and REG trials (14.9% and 13.1%, respectively). The PACK condition also reduced VO(2)max, but to a lesser extent (4.8 +/- 5.3%). At VO(2)max, VE was decreased and expiratory mouth pressure and external breathing resistance (BR) were increased in both the SCBA and REG trials. There was a significant correlation between the change in maximal V(E)and VO(2)max with the SCBA. The results show that the SCBA reduces VO(2)max by limiting V(E) secondary to the increased BR of the SCBA regulator.