Our understanding of the development of children and adolescents' aerobic fitness is limited by ethical considerations and methodological constraints. Protocols, apparatus, and criteria of maximal effort used with adults are often unsuitable for use with children. In normal children and adolescents, peak VO2 increases with growth and maturation, although there are indications that girls' peak VO2 may level off around 14 years of age. Males exhibit higher values of peak VO2 than females, and the sex difference increases as they progress through adolescence. The difference between males and females has been attributed to the boys' greater muscle mass and hemoglobin concentration. It appears that boys experience an adolescent growth spurt in peak VO2, which reaches a maximum gain near the time of PHV, but data are insufficient to offer any generalization for girls. Peak VO2 has usually been expressed in relation to body mass, and with this convention it appears that boys' values are consistent throughout the developmental period, whereas girls' values decrease as they get older. This type of analysis may, however, have clouded our understanding of growth and maturational changes in peak VO2, and scaling for differences in body size may provide further clarification. If differences are shown where none were previously thought to exist, then physiological explanations must be sought. Methodological issues have also hindered the understanding of how children's blood lactate responses to exercise develop. The actual lactate level recorded during an exercise test is influenced by the site of sampling and the blood handling and assay techniques. Valid interstudy comparisons can only be made where similar procedures have been employed. In general, children demonstrate lower blood lactate levels at peak VO2 than adults, although individual variation is wide. Therefore the use of blood lactate measures to confirm the attainment of peak VO2 cannot be supported. Exercise at the same relative submaximal intensity elicits a lower blood lactate in children than in adults, but interpretation and identification of developmental and maturational patterns of response are limited by the use of different testing conditions and reference points (e.g., lactate threshold and fixed level reference points). There is growing evidence that the 2.5 mM reference level should be used in preference to the 4.0 mM level, as the adult criterion occurs close to maximal exercise in many children and adolescents. Explanations for child-adult differences in blood lactate responses to exercise are difficult to elucidate.(ABSTRACT TRUNCATED AT 400 WORDS)