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Factors influencing the growth of stature, body mass, and physical performance have been examined in a sample of 546 children participating in the Trois-Rivieres regional study of growth and development (2095 anthropometric observations, 3993 performance test measurements). Subjects were sub-divided into approximately equal-sized subgroups on the basis of sex, milieu (urban or rural-industrial) and program (control, receiving normal 40 min of physical education per week, and experimental, receiving a nominal 5 hr of specialized physical education per week). Grouping anthropometric data by birthdate and thus season of testing, stature deviated by some 0.8 cm about mean annual values, growth being least for children having birthdays in the period April-June; such differences were about twice as large in the group receiving enhanced physical education as in the control series.Body mass, similarly grouped, deviated by about 0.6-0.7 kg about mean annual values. Skinfold readings did not change, suggesting the variation of mass occurred in lean tissue. The alteration of mass was of the order predicted from changes in stature, although height/weight loops showed some hysteresis, particularly in control subjects. Scores for all 6 CAHPER (Canadian Association for Health, Physical Education and Recreation) performance tests improved from October to June, and deteriorated from June to October. Assuming the birthdate anthropometric measurements reflect seasonal variations in the growth of our entire sample of children, the performance changes are the opposite of what would be predicted from dimensional theory. It is thus suggested that the school physical education program may have a specific, influence upon performance. Seasonal differences of CAHPER test scores were twice as large as normal in those students who received the enhanced program of physical education.