Endurance of the inspiratory muscles was measured in normal volunteers using a threshold resistance that produced a relatively constant mouth-pressure load, independent of inspiratory flow rate (VTI). Breathing pattern was controlled by visual feedback from an oscilloscope. Endurance was measured as the length of time (Tlim) a target VTI could be maintained with maximum effort. Effects of changes in breathing pattern on Tlim were compared with control measurements made the same day. Increases in VTI or in duty cycle (inspiratory time/total period) shortened Tlim, whereas decreases lengthened Tlim. However, effects of changes in VTI were less than equivalent changes in tidal volume produced by alterations in duty cycle. Furthermore, when two breathing pattern changes were altered simultaneously to keep the rate of external inspiratory work (Winsp) constant, significant effects due to changes in duty cycle were still observed. In conclusion, 1) both VTI and duty cycle have significant effects on measurements of inspiratory muscle endurance and 2) the effects of VTI are less than the effects of duty cycle for the same Winsp.