A new method of aerobic threshold (AerT) determination, as a minimum of so-called lactate equivalent (Berg et al. 1980; Lehmann et al. 1983), was compared with the generally used method (AerT1) presented first by Wasserman et al. (1973). The comparison was made with data from 32 subjects in repeated maximal exercise tests on a bicycle ergometer. The AerT as a minimum of lactate equivalent was determined using both a computerized method for curve fitting (AerT3) and visual inspection (AerT2). AerT2 and AerT3 showed similarity: the regression coefficient was 0.88 and 0.95 and the correlation coefficient was 0.84 and 0.88 in the first and in the second tests, respectively. AerT1 and AerT2 showed similar mean estimated values, but AerT3 showed slightly higher mean values than AerT1 (p less than 0.05 and p less than 0.01) or AerT2 (p less than 0.01). The regression coefficients between AerT1 and AerT2 or AerT3 (0.60-0.71) showed different slopes from the identity (p less than 0.001). The corresponding correlation coefficients were 0.88 and 0.89 with AerT2, and 0.75 and 0.79 with AerT3 respectively in the first and second tests. However, in two thirds of all determinations the difference between these two estimation methods (AerT1 and AerT2) was less than or equal to 0.10 l O2.min-1. In conclusion, AerT as a minimum of lactate equivalent differed slightly from AerT determined using the traditional method. However, its determination criterion is unambiguous and allows objective computerized curve fitting. The physiological correlates of this new AerT index should be studied carefully before final evaluation of its usefulness in applications can be made.