Height and maximal aerobic power (VO2 max) were analysed longitudinally in 14 active and 11 inactive boys from the Saskatchewan Longitudinal Growth Study. VO2 max values were obtained from a treadmill run repeated each year. The Preece-Baines Model I was fitted to height and VO2 max values over the duration of the study. No significant differences were found for height attained between the two groups. VO2 max comparisons revealed no significant differences before the take-off point for the adolescent growth spurt. Significant differences occurred in the VO2 max attained by the age of peak adolescent velocity in VO2 max, the increase from take-off to peak and in the adult value, with active boys having higher values. It is concluded that activity before adolescence causes no significant increase in VO2 max, but that adolescence is the critical period during which consistently higher rates of increase in the VO2 max of active boys result in a significantly greater adult value.