Insufficient respiratory muscle endurance (RME) may be one of the factors limiting ventilation during peak athletic performance. Our purpose was to determine whether the RME of highly trained cyclists could be enhanced and if so, to determine the effects of improved RME on their maximal exercise performance. Ten male cyclists (maximal oxygen consumption (VO2max) greater than 60 ml/kg-1) began the study by performing 3 tests. These were VO2max, RME measured as maximal sustainable ventilatory capacity (MSVC) and maximal exercise endurance (tlim) measured by an endurance cycling test to exhaustion at 90% of their maximal power output. Five subjects then completed 4 weeks of isocapnic hyperpnea training (16 session) and 5 subjects were controls. Following this training interval, each subject repeated the initial tests. After the RME training, the MSVC increased from 155 +/- 11 to 174 +/- 12 l/min (p = 0.004) for the training subjects while there was no change in the controls (155 +/- 26 and 150 +/- 34 l/min). There were no changes for any of the 10 subjects in either the maximal exercise performance (VO2max = 66.1 +/- 4.7 to 66.5 +/- 4.8 ml.kg-1) or the maximal exercise endurance (tlim = 335 +/- 79 to 385 +/- 158 sec). In conclusion, 4 weeks of respiratory muscle endurance training increased respiratory muscle endurance but had no effect on the maximal cycling performance of highly trained cyclists.