The purpose of this study was to determine whether or not there are preexisting holes in the endothelial and epithelial basal laminae of alveolar walls and to determine the path taken by neutrophils as they migrate from the capillaries to the airspace of the alveoli during inflammation. Using transmission electron microscopy and serial thin sections of normal rabbit and mouse lung, we have demonstrated the presence of slit-like holes in the capillary basal laminae and round holes in the basal laminae of type 2 pneumocytes. The slits in the capillary basal laminae were observed at the intersection of the thick and thin walls where endothelium, pericytes, and fibroblasts make close contact. The round holes in the type 2 cell basal laminae were observed at sites of close contact with fibroblasts. Neutrophils were observed to migrate through these slits and holes during streptococcal pneumonia in rabbit lungs. We conclude that during inflammation in the lung, migrating neutrophils displace pericytes and fibroblasts from the slits in the capillary basal lamina and then crawl through these slits into the alveolar interstitium. We postulate that neutrophils find their way to type 2 pneumocytes by following interstitial fibroblasts. We believe that neutrophils displace fibroblasts from their close contacts with the type 2 cells and then crawl through the holes in the basal lamina into the basal lateral space of the type 2 cells. From there, neutrophils migrate into the alveolar airspace.