Thirty-two elite junior athletes in two age categories, older than or equal to 15 years old (O15) (8 females and 9 males) and less than 15 years old (U15) (8 females and 7 males), performed a laboratory-based duathlon (run-ride-run). At the completion of the event, significant body mass losses were recorded for all groups. Compared with the other three groups, the O15 males lost body mass at a greater absolute rate (1.26 +/- 0.06 kg.hr-1 vs. a mean of 0.62 +/- 0.11 kg.hr-1 for the other three groups) and a greater relative rate (1.95 +/- 0.10%BM.hr-1 vs. a mean of 1.23 +/- 0.19%BM.hr-1 for the other three groups) (p < .05). No differences were observed between groups for fluid consumption. Subjects consumed more fluid (p < .05) during the cycle phase and postevent than preevent or during the run phases. Results indicated that the athletes' fluid intake practices were insufficient to maintain adequate hydration during the simulated event.