Minor differences in lung growth and development during childhood have been considered as a potential cause of rapid decline in pulmonary function in adulthood. Inhalation of ozone commonly causes changes in both body weight and lung volumes, which complicates interpretation of any changes in lung growth. The effects of ozone on lung growth were studied in rats which were pair fed. This technique permitted comparison of ozone-exposed and filtered-air control rats of the same body weight and body size as well as age and sex. Exposure was to filtered air or to 0.64 or 0.96 ppm ozone (UV standard) 8 h/night for 42 nights. A second control group was fed ad libitum and exposed to only filtered air. Half the rats were studied at the end of the 42-night exposures, the rest after a 42-day post-exposure period during which all rats were fed ab libitum and breathed filtered air. Rats examined at the end of the exposure period had larger saline and fixed lung volumes. These larger lungs had greater volumes of parenchyma, alveoli and respiratory bronchioles. Some of these changes persisted throughout a 42-day post-exposure period. Ozone inhalation by young rats alters lung growth and development in ways likely to be detrimental and those changes persist after ozone inhalation stops.