BACKGROUND: TauVO(2 )at the onset of constant work rate (CWR) exercise is a variable of aerobic fitness that shortens with physical training and lengthens with cardiopulmonary disease. Determination of tauVO(2) with sufficiently high confidence has typically required multiple exercise transitions limiting its clinical application. OBJECTIVES: To design a protocol to determine tauVO(2) reliably but simply. METHODS: On each of three days, five healthy men performed two CWR tests on a cycle ergometer below the metabolic threshold (VO(2)theta) for blood lactate accumulation as determined by gas exchange measurements followed by an incremental work rate (IWR) test. TauVO(2) was determined (a) from the on-transit (on-tauVO(2)) and off-transit (off-tauVO(2)) of six CWR tests both individually and superimposed, using non-linear regression with a monoexponential model, and (b) by geometric analysis of the IWR tests (ramp-tauVO(2)). RESULTS: Group means (SD) were: VO(2)max 3.84 (0.44) litres/min, VO(2)theta 1.88 (0.23) litres/min, steady state exercise VO(2) 1.67 (0.07) litres/min, on-tauVO(2) 38.0 (5.3) seconds, off-tauVO(2) 39.0 (4.3) seconds, and ramp-tauVO(2) 60.8 (15.4) seconds. On-tauVO(2) correlated with off-tauVO(2) (r = 0.87), VO(2)max (r = -0.73), and VO(2)theta (r = 0.89). The pooled mean tauVO(2) from six superimposed tests agreed with the arithmetic grand mean of the six tests. CONCLUSIONS: The average of on-tauVO(2) and off-tauVO(2) fell within the 95% confidence interval of the pooled mean by the second test. Ramp-tauVO(2) was longer and less reproducible. These findings support the use of both on- and off-transit data for the determination of tauVO(2), an approach that reduces the number of transitions necessary for accurate determination of tauVO(2), potentially enhancing its clinical application.