The objectives of this study were to: (a) develop a physiological profile for a group of trained triathletes and (b) determine whether multiple modes of training result in general or specific adaptations. VO2max of 13 trained triathletes (mean = 29.5 yr) was measured during treadmill running (TR), cycle ergometry (CE), and tethered swimming (TS) over a 6-wk period encompassing a half-triathlon (1.2 mile swim/56 mile bike/13.1 mile run). Most subjects performed two tests in each mode. Since test-retest reliability coefficients for TR, CE, and TS VO2max were 0.97, 0.93, and 0.97, respectively, results were averaged: formula; see text The mean TR VO2max indicated that the subjects were well-trained, but not of elite caliber. Mean CE VO2max was 95.7% of the TR value, which is greater than the value typically found in non-cyclists (88 to 92%) but less than that of highly trained cyclists (98 to 105%). Mean TS VO2max was 86.6% of the TR value. As in cyclists, this percentage is greater than that of recreational swimmers (78 to 82%) but less than that of elite swimmers (93 to 95%). Running and cycling times in the triathlon were significantly (P less than 0.01) related to the corresponding VO2max values (r = -0.68 and r = -0.78, respectively), but swimming times were not (r = -0.50). It is concluded that these triathletes were well-trained in all events, but not to the same extent as athletes who train in only one sport. Running and cycling performance were associated with VO2max.(ABSTRACT TRUNCATED AT 250 WORDS)