Spironmetric measurements of lung function were made on six or more occasions over a five week period in 1982 in 39 children attending a summer day camp in Mendham, N.J. Mean slopes of the linear regressions of FVC, FEV1, MMEF and PEFR vs. peak 1 hr 03 concentrations for the day of the function measurements in the individual children were negative, with PEFR showing the strongest effect. There were no consistent associations with other environmental variables. Analysis of residuals from the regression lines for PEFR vs. daily 1 hr O3 peak and 4 hr H+ and SO4= exposures for each child showed large overestimates in PEFR during the second week of the study. There was a photochemical smog episode immediately preceding the second week, with successive daily 1 hr O3 peaks of 143, 185, 165, 135 and 115 ppb, and this smog exposure appears to be the most likely cause of the persistent second week depression in PEFR from the values expected on the basis of an overall regression from the five weeks of data.