The purpose of this investigation was to compare the physiological responses in cycling and the energy cost (EC) of running after cycling in elite junior (J (male) and J (female)) and senior (S (male) and S (female)) triathletes and to determine the relationship between laboratory physiological parameters and performance in an elite "standard" distance triathlon. Thirty-one elite triathletes competing at World Championship level (age: 23.4 +/- 4.8 y; height: 172.6 +/- 6.8 cm; body mass: 64.4 +/- 7.2 kg; V.O (2)max = 67.8 +/- 8.3 ml x kg (-1) x min (-1)) comprising J (male) (n = 7), J (female) (n = 6), S (male) (n = 9) and S (female) (n = 9) athletes performed a laboratory trial that consisted of submaximal treadmill running (to determine EC), maximal then submaximal cycle ergometry (to determine the peak power output [PPO], V.O (2)max, the ventilation threshold [VT] and cycling economy) followed by an additional submaximal running bout. Swimming, cycling, running and overall race performance (min) over a standard event was also measured in the field. S (male) had a faster cycle, run and overall triathlon times than J (male). S (female) demonstrated a faster cycle and overall triathlon time than J (female). The V.O (2)max (74.7 +/- 5.7 vs. 74.3 +/- 4.4 and 60.1 +/- 1.8 vs. 61.0 +/- 5.0 ml x kg (-1) x min (-1)) and cycling economy (72.5 +/- 4.5 vs. 73.8 +/- 4.3 and 75.6 +/- 4.5 vs. 79.8 +/- 9.8 W x l (-1) x min (-1)) were similar between the junior and senior, in both male and female triathletes. However, S (female) possessed a significantly higher PPO than J (female). S (male) had a higher VT (%V.O (2)max) than J (male) whereas the VT was similar in J (female) and S (female). There were no significant differences in EC change from the first to the second running bout between J (male) and S (male), whereas, in contrast, J (female) exhibited a significantly (p < 0.05) higher difference in EC than S (female). When all subjects were pooled, the overall triathlon time (min) was significantly correlated to V.O (2)max (r = -0.80; p < 0.001) and PPO (W) (r = -0.85; p < 0.001) in cycle ergometry. In conclusion, elite senior triathletes can be distinguished from their younger (junior) counterparts, mainly by a higher PPO in cycling and a lower increase in the whole body energy cost of running after cycling in female and by a higher ventilatory threshold in male triathletes.