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Newborn Swiss‐Webster mice were exposed to intermittent 0.3 ppm ozone (O 3 ) for 6 wk, and the lungs of 137 exposed and 138 control animals (275 of the 301 in the colony) were suitable for computer‐assisted image‐analysis quantitation. Data were obtained on numbers and areas of lactate dehydrogenase stained type II cells, and also area, perimeters, and linear intercepts of the alveolar wall. As noted in an earlier study of adult mice, ozone exposure increased all cell and wall measurements. In contrast to the adult animal findings, there was a greater increase in mean type II cell area (p = 0.07) than in numbers of type II cells, with the latter increase falling short of statistical significance. The increase in cell area at the expense of cell number may largely be due to cell pairing or multicell clustering, events that would mask type II cell hyperplasia. Ozone effects on the type II cell population implicate damage to the type I alveolar lining cells. Moreover, the increases in alveolar wall measurements that were found in both the adult and developing mouse lung imply an alteration of the lung scaffolding, and this raises the question of impaired regeneration of the epithelial lining. [ABSTRACT FROM PUBLISHER]