The purpose of this study was to determine the effects of different inspiratory resistances on exercise performance and to describe physiological and subjective responses during constant load work of an intensity that elicits the maximum sensitivity to respiratory factors of respirator wear. Nine subjects (mean age 25.8 +/- 4.8 years) performed exhaustive treadmill exercise during constant load work of 80% of maximal aerobic capacity while inspiring against one of five randomly assigned inspiratory resistance conditions. For inspiratory resistances of 0.20 kPa (R1), 0.29 kPa (R2), 0.39 kPa (R3), and 0.49 kPa (R4), measured at a steady air flow rate of 85 L min-1, average performance times were 84, 75, 54, and 30% of the control condition. Performance time reductions were significant for R3 and R4 trials. Results also indicated a significantly reduced mean respiration rate, minute ventilation, and oxygen ventilatory equivalent (VE/VO2) for the R4 condition at exercise breakpoint compared to control. For resistances R1-R4, mean VE/VO2 were significantly reduced from control by approximately 12, 17, 19, and 31%. At breakpoint, mean ratings of perceived exertion were similar for control, R1, R2, and R3 conditions but were significantly lower for R4 versus control. Breathing comfort did not differ significantly between resistances below the R3 level, but discomfort was significantly greater for conditions R3 and R4. These findings suggest that the impact of the respiratory limitations of respirators can be diminished by employing respirators with inspiratory resistances below the R3 level.