The time course of changes in blood lactate concentration and ventilatory gas exchange was studied during an incremental exercise test on a cycle ergometer to determine if the lactate accumulation threshold (LT2) could be accurately estimated by the use of respiratory indices (VT2) in young athletes. LT2 was defined as the starting point of accelerated lactate accumulation. VT2 was identified by the second exponential increase in VE and the ventilatory equivalent for O2 uptake with a concomitant nonlinear increase in the ventilatory equivalent for CO2 output. Twelve trained subjects, aged 18-22 years, participated in this study. The initial power setting was 30 W for 3 min with successive increases of 30 W every minute except at the end of the test when the increase was reduced. Ventilatory flow (VE), oxygen uptake (VO2), carbon dioxide output (VCO2), and ventilatory equivalents of O2 and CO2 were determined during the last 30 s of every minute. Venous blood samples were drawn at the end of each stage of effort and analysed enzymatically for lactate concentration. After each test, LT2 and VT2 were determined visually by two investigators from the graphic results using a double-blind procedure. The results [mean (SEM)] indicate no significant difference between LT2 and VT2 expressed as VO2 [43.98 (1.70) vs 44.93 (2.39) ml.min-1 x kg-1], lactataemia [4.01 (0.28) vs 4.44 (0.37) mM.l-1], or heart rate [171 (3.36) vs 173 (3.11) min-1]. In addition, strong correlations were noted between the two methods for VO2 (r = 0.90, P < 0.001), lactataemia (r = 0.75, P < 0.01), and heart rate (r = 0.96, P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)