The aim of this study was to assess the use of the 4 mM blood lactate level measured during incremental treadmill exercise as an indicator of exercise performance in 11-13 year old children. Fifty girls and 53 boys (means age 12.2 +/- 0.6 years) gave informed consent to participate in the study. The children completed a discontinuous, incremental running test on a motorized treadmill. A 1 min pause separated the 3 min exercise stages during which time duplicate capillary blood samples were taken for immediate assay of whole blood lactate concentration. Cardiorespiratory variables were measured continuously during exercise using a computerized on-line system. Mean values for peak oxygen uptake were 48 +/- 7 ml kg-1 min-1 and 42 +/- 7 ml kg-1 min-1 in boys and girls respectively. The exercise intensity corresponding to a blood lactate concentration of 4 mM was interpolated from plots of % peak oxygen uptake vs blood lactate and was reached at 91 +/- 7% peak oxygen uptake in both boys and girls. The results indicate that children can exercise at intensities close to their peak oxygen uptake without accumulating high levels of blood lactate and the use of the 4 mM blood lactate level to assess and monitor exercise performance in children of this age may therefore be inappropriate.