Our recently reported animal (rat) model of maternal cigarette smoke exposure during pregnancy is characterized by fetal growth retardation and lung hypoplasia. We have further defined the fetal lungs using quantitative histologic techniques. Compared to controls, lung volume at term is reduced in the experimental animals (0.28 versus 0.33 ml, p less than 0.005). Saccules (fetal" alveoli") are reduced in number (3 X 10 (6) versus 5.5 X 10 (6), p less than 0.005) and increased in size (average saccular volume: 35 X 10 (-9) versus 21 X 10 (-9) ml, p less than 0.025). These changes in size and number are consequences of reduced formation of saccule partitions (septal crests) in the experimental lungs (volume density: 0.013 versus 0.018, p less than 0.025). Internal surface area is decreased in the hypoplastic lungs (161 versus 198 cm2, p less than 0.001). The total length of parenchymal elastic tissue is diminished (224 versus 354 M, p less than 0.05). In short the hypoplastic lungs contain fewer, larger saccules and the surface potentially available for gas exchange is reduced. These results show that maternal smoking in rats adversely modifies fetal lung growth. If these observations are applicable to humans, then reduced lung growth in children of smoking mothers may begin antenatally.