Controversial literature exists concerning the occurrence of inspiratory muscle fatigue during efforts performed outside a laboratory. The purpose of this study was to assess inspiratory muscle strength and endurance time measured as Tlim, the length of time a subject can endure a task before the onset of fatigue, in ultra-marathon runners by simple non-invasive techniques before and after an ultra-marathon (87 km). Ten runners, (8 males and 2 females), who had normal clinical evaluation and lung function underwent inspiratory muscle assessment by measurement of maximal inspiratory mouth pressures and sustained inspiratory mouth pressures at a given target pressure and a given duty cycle. The measurements were performed prior to the race and 3 days after the race. No significant difference was observed in the inspiratory muscle strength 3 days after the race (p > 0.37), but the inspiratory muscle endurance time as measured by Tlim, was significantly lower (p < 0.002), with an overall decrease of 26.5%. Inspiratory muscle strength was normal 3 days after the race, however the inspiratory muscle endurance time as measured by sustained inspiratory pressure was still impaired 3 days after the race.