Perceived exertion has been investigated during incremental exercise to exhaustion with athletes, according to the 6-20 scale proposed by Borg (RPE, 1970) and a new scale based on the estimation of exhaustion time (t(lim)) (Estimated Time Limit or ETL, from 1 to 20; ETL= 21 - 2 n, with n = log2t(lim)). ETL increased linearly with the percentage of maximal aerobic power (%MAP) up to the ventilatory threshold (VT). Beyond VT, an inflection of the ETL-%MAP relationship was observed. RPE and ETL calculated at VT were equal to 15 +/- 1.7, i.e. an exercise intensity perceived as "hard", and 10 +/- 2.3, i.e. an estimated exhaustion time equal approximately to 45 minutes. Standard deviations for RPE and ETL at VT were too large for an accurate estimation of this threshold. The results of the present study suggest that RPE and ETL should be used in addition to physiological data, but not replace them in the prescription of a training program.