Respiratory functions were measured daily by spirometry over four weeks at a summer camp in northwestern New Jersey. Multiple regression analyses indicated that O3 concentration, cumulative daily O3 exposure, ambient temperature, and humidity were the most explanatory environmental variables for daily variations in function, and that O3 concentration had the strongest influence on FVC, PEFR, and MMEf. For FEV1, cumulative daily O3 exposure and heat stress had greater relative effects. Linear regressions were performed for each child between O3 concentration and function, and all average slopes were significantly negative (p <0.05) for PVC, FEV1, PEFR and MMEF for all children, and for boys and girls separately. The implications of these short-term effects are unknown. However, the results in these free living children are comparable to those found in chamber studies with comparable exposures.