The influence upon muscle strength of 1 h/day of required physical education was tested in 546 prepubescent children over a 5-year period. Experimental students began the program in Grade 1, with immediately preceding and succeeding classes serving as controls. Annual measurements showed a linear increase of limb circumferences with stature (H), but strength increased more rapidly than H². Strength data showed gender effects (M > F) at all ages and environmental effects (rural > urban) in older children (10–12 years). Experimental students were stronger in 19 of 42 comparisons. Girths were greater in girls (7 of 18 comparisons) and in the rural environment (4 of 18 comparisons), but were unaffected by the experimental intervention. Bone diameters were greater in the boys (16 of 18 comparisons) and in the urban environment (2 of IS comparisons). Daily required physical education leads to small increases of isometric strength without significant increments of limb dimensions.