Eleven male triathletes were studied to determine the relationships between selected metabolic measurements and triathlon performance. Measurements of oxygen uptake (VO2), pulmonary ventilation (VE), and heart rate (HR) were made during submaximal and maximal 365.8 m freestyle swimming (FS), cycle ergometry (CE), and treadmill running (TR). Submaximal workloads were 1 m/s for swimming, 200 W for cycling, and 201.2 m/min for running. The mean VO2 max (l/min) was significantly (p less than .05) lower during FS (4.17) than CE (4.68) or TR (4.81). Swimming, cycling, and running performance times during the Muncie Endurathon (1.2 mile swim, 56 mile cycle, 13.1 mile run) were not significantly related to the event-specific VO2 max (ml/kg/min): -.49, -32 and -.55, respectively. The VO2 max expressed in l/min was found to be significantly (p less than .05) related to cycling time (r = -.70). A significant (p less than .05) relationship was observed between submaximal VO2 (ml/kg/min) during TM and run performance time (r = .64), whereas swimming and cycling performance times were significantly (p less than .05) related to submaximal VO2 max (l/min), r = .72 and .60, respectively. The percentage of VO2 (%VO2 max) used during the submaximal tests was significantly (p less than .05) related to swimming (.91), cycling (.78), and running (.86) performance times. Time spent running and cycling during triathlon competition was significantly (p less than .05) related to overall triathlon time, r = .97 and .81, respectively. However, swimming time was not significantly related (.30) to overall triathlon time. This study suggests that economy of effort is an important determinant of triathlon performance.