This study was designed to examine differences in the respiratory health status of preadolescent school children, aged 7-11 years, who resided in 10 rural Canadian communities areas of moderate and low exposure to regional sulfate and ozone pollution. Five of the communities were located in central Saskatchewan, a low-exposure region, and five were located in southwestern Ontario, an area with moderately elevated exposures resulting from long-range atmospheric transport of polluted air masses. In this cross-sectional study, the child's respiratory symptoms and illness history were evaluated using a parent-completed questionnaire, administered in September 1985. Respiratory function was assessed once for each child in the schools between October 1985 and March 1986, by the measurement of pulmonary function for forced vital capacity (FVC), forced expiratory volume in 1 sec (FEV1.0), peak expiratory flow rate (PEFR), mean forced expiratory flow rate during the middle half of the FVC curve (FEF25-75), and maximal expiratory flow at 50% of the expired vital capacity (V50max). The 1986 annual mean of the 1-hr daily maxima of ozone was higher in Ontario (46.3 ppb) than in Saskatchewan (34.1 ppb), with 90th percentile concentrations of 80 ppb in Ontario and 47 ppb in Saskatchewan. Summertime 1-hr daily maxima means were 69.0 ppb in Ontario and 36.1 ppb in Saskatchewan. Annual mean and 90th percentile concentrations of inhalable sulfates were three times higher in Ontario than in Saskatchewan; there were no significant differences in levels of inhalable particles (PM10) or particulate nitrates. Levels of sulfur dioxide (SO2) and nitrogen dioxide (NO2) were low in both regions. After controlling for the effects of age, sex, parental smoking, parental education, and gas cooking, no significant regional differences were observed in rates of chronic cough or phlegm, persistent wheeze, current asthma, bronchitis in the past year, or any chest illness that kept the child at home for 3 or more consecutive days during the previous year. Children living in southwestern Ontario had statistically significant (P < 0.01) mean decrements of 1.7% in FVC and 1.3% in FEV1.0 compared with Saskatchewan children, after adjusting for age, sex, weight, standing height, parental smoking, and gas cooking. There were no statistically significant regional differences in the pulmonary flow parameters (P > 0.05).