Offspring of control and experimental (chronically exposed to whole cigarette smoke) rats were sacrificed on 15th postnatal day. Pulmonary tissues were processed for quantitative electron microscopic analyses. Arithmetic and harmonic mean thicknesses were directly measured and diffusion capacity was derived for separate layers (tissue, plasma, erythrocyte) and the entire lung. Experimental offspring showed a greater volume density of interstitium in parenchyma (P less than 0.04), arithmetic (P less than 0.05) and harmonic (P less than 0.009) mean thicknesses. Diffusion capacities for lungs of both animal groups, however, were similar (0.00543 and 0.0054 cm3 O2.min-1.mm Hg-1.g body weight-1 for control and experimental lungs respectively). Experimental tissue appeared better adapted for optimal exchange of gases (P less than 0.05), as indicated by corrugation index (ratio between arithmetic and harmonic mean thicknesses) than for the control counterpart. In spite of induced alterations in several developmental processes by a presently used experimental procedure, tissue capacity for transport of gases was not significantly changed in either the alveolo-capillary membrane or the entire lung.