The aims of this study were, in a group of adolescents, firstly to identify the absolute heart rates (HR) and the percentages of maximal heart rates (HRmax) corresponding to 40%, 60% and 80% of peak oxygen uptake (PVO2), secondly to identify absolute and relative (%PVO2) oxygen uptakes (VO2) corresponding to HR of 120, 140 and 160 beats.min-1, and thirdly to examine a possible effect of fatness and fitness on the relationship between HR and VO2. The subjects were 127 (60 boys, 67 girls) adolescents with a mean age of 14.8 (SD 0.3) years. The HR and VO2 were measured by means of an incremental exercise test to exhaustion. Linear regressions were performed for the HR-VO2 and VO2-HR relationships using absolute and relative (%HRmax, %PVO2) data for each individual. From these regressions, target HR and VO2 were computed. Average target HR corresponding to 40%, 60% and 80% of PVO2 were: 119 (SD 9), 145 (SD 9), 171 (SD 8), and 120 (SD 10), 146 (SD 8), 172 (SD 8) beats.min-1 for boys and girls, respectively. Average VO2 corresponding to HR of 120, 140 and 160 beats.min-1 were: 22 (SD 5), 30 (SD 5), 38 (SD 6) and 18 (SD 4), 24 (SD 4), 31 (SD 4) mlO2.kg-1.min-1 for boys and girls, respectively. An analysis of covariance showed a significant fitness effect (P < 0.001) for predicted VO2 at all HR studied. The results suggest that the use of absolute HR to define exercise intensity levels when assessing young people's physical activity using HR monitoring detracts from the validity of the interpretation of the data.