We assessed the effect of an endurance triathlon consisting successively of a 3.8-km swim, a 180-km bicycle ride, and a 42-km run on the forced expiratory spirogram, indices of inspiratory and expiratory muscle strength (PImax and PEmax), and maximal voluntary ventilation (MVV). Twelve male participants were recruited from competitors in the Cape Cod Endurance Triathlon and underwent studies on the afternoon before the event, after each segment, and on the following morning. Participants averaged 32.9 yr of age. All completed the triathlon with an average finishing time of 12 h 45 +/- 90 min. Following completion of the triathlon, statistically significant declines occurred in FVC (7.1%), FEV1 (8.4%), FEF25-75% (15.2%), and FEF50% (18.6%), but not in MVV. On the morning after the triathlon, only FEV1 remained significantly below baseline. PImax was not significantly reduced after the swim, but significant reductions did occur after the bicycle and running events (26% and 25%, respectively); full recovery had occurred by the following morning. PEmax did not change significantly. We conclude that vital capacity, flow rates at mid-lung volumes, and inspiratory muscle strength decline as a consequence of participation in a triathlon.