MCLELLAN, T. M. and G. C. CASS. Metabolic and cardiorespiratory responses relative to the anaerobic threshold.Med. Sci. Sports Exerc, Vol. 21, No. 2, pp. 191–198, 1989. The present study has compared the metabolic and cardiorespiratory responses for two groups of male subjects during 20 min of exercise at the anaerobic threshold (AT), at AT+1/3, and at AT+2/3 of the difference (Δ) between AT and &OV0312;O2max. A log-log transformation of the lactate (LA)-power output relationship was used to define AT and divide subjects into a high (N= 7, AT = 51.9 ± 1.5% &OV0312;O2max) and low (N= 5; AT = 41.9 ± 1.8% &OV0312;O2max) AT group. No differences were observed between groups during exercise at AT for &OV0312;E·&OV0312;O2-1, &OV0312;E·&OV0312;CO2-1pH, pCO2, blood LA, and plasma strong ions Na+, K+, and Cl-. Although blood LA values were significantly elevated for the low AT subjects (2.3 ± 0.6 mmol·l-1) compared with the high AT group (1.0 ± 0.1 mmol·l-1) during exercise at AT+1/3Δ, no other differences between groups were noted. In contrast, marked differences were observed between groups during exercise at AT+2/3Δ. The high AT group showed no change in &OV0312;E(79.1 ± 4.8 1·min-1), pH (7.367 ± 0.01), pCO2(37.3 ± 1.2 mm Hg), and blood LA (2.9 ± 0.3 mmol·l-1) during the final 10 min of the 20 min exercise test. The low AT group, however, showed a progressive increase in &OV0312;E(from 88.7 ± 4.2 to 108.0 ± 8.0 1·min-1) and blood LA (from 8.6 ± 1.0 to 11.4 ± 2.7 mmol·l-1) and a progressive decrease in blood pH (from 7.294 ± 0.014 to 7.251 ± 0.030) and pCO2(from 35.2 ± 1.2 to 29.2 ± 1.3 mm Hg) from minute 10 to minute 20 of exercise. These data suggest that AT and &OV0312;O2maxdo not provide common reference points for calculating exercise intensity and/or that a metabolic rate exists above AT and below &OV0312;O2maxthat defines an individual's maximal steady state for the cardiorespiratory and metabolic response to exercise.