The purposes of this study were to provide baseline data on the peak oxygen consumption (VO2) of British children, aged 11-16 years and to examine the peak VO2 of children in relation to their pubertal stage of development. The peak VO2 of 226 boys and 194 girls was determined during either treadmill running or cycle ergometry. The sexual maturity of 320 of the children was estimated using Tanner's indices. Peak VO2 increased with chronological age in both sexes and from about the age of 12 years boys exhibited significantly higher (P less than 0.05) values than girls. Boys' peak VO2 in relation to body mass was consistent over the age range studied and was superior (P less than 0.05) to girls' values at all ages. It appears that mass-related peak VO2 is independent of sexual maturity in both sexes. The more mature boys demonstrated a significantly higher (P less than 0.05) peak VO2 (1.min-1) than the less mature boys on both ergometers. The more mature girls demonstrated significantly higher (P less than 0.05) peak VO2 (1.min-1) than the less mature girls only on the cycle ergometer. On both ergometers the differences between the peak VO2 of the girls and boys were more pronounced in the mature children whether expressed in relation to body mass or not. Comparison of the results with earlier data drawn from smaller samples failed to provide evidence to suggest that British children's peak VO2 has declined in recent years. No study with which to compare our maturity peak VO2 data appears to be available.