A breathing apparatus with a partial failure may have higher breathing resistance (R) than expected or a breathing apparatus may be needed for harder work than it was designed for. The effects of very high R on physical endurance and on breathing was not known. Fifteen subjects took part in this IRB approved study to determine such effects during moderate exercise (60 of peak O2 consumption) on a cycle ergometer on dry land at sea level. R was such that the work of breathing per volume (volume-averaged pressure) ranged from nominal 3 to 9 kPa (J/L), i.e. up to 3 times higher than NEDUs limits for diving. Individuals exercise endurance varied greatly. With the least high R, it ranged from 4.5 min to the protocols maximum 60 min, with the highest R two subjects continued for 60 min, while one other exercised for less than 2 min. The endurance time for the 90th percentile was 12 min at the lowest R and 3 min for the highest. In general, the minute ventilation decreased (reduced breathing frequency, unchanged tidal volume and duty cycle) with increasing R and the end-tidal CO2 values increased, some subjects reaching levels close to 8 of the dry gas (57 mm Hg). No subject reached the abort limit of 65 mm Hg. Some subjects who maintained high CO2 levels reported no or low dyspnea. Rating of perceived exertion did not correlate with R. Reactions to very high R are not predictable. Low scores for dyspnea or perceived exertion do not indicate acceptable R. NEDUs limits for R in a divers breathing apparatus cannot be used at sea level. Values for R found in simulated or real failures of breathing apparatus can be used with the endurance times found here to judge likely endurance times.