The aim of the study was to determine the destination of the alpha 1-antitrypsin-elastase complex, which is found in circulating blood after the peroral administration of elastase. The complex was made in vitro by mixing hog pancreatic elastase with human alpha 1-antitrypsin and then injected intravenously into rats and mice. Tissues taken at various times after injection were subjected to histochemical staining using an antibody against elastase. Light microscope observations revealed dense deposition of reaction products in the elastic lamina of the arterioles; moderate or slight deposits were seen in the tissues surrounding arteries, in the tubular epithelial cells of the proximal convoluted tubules in the kidney, and in the pancreatic ducts. Immunoelectron microscopy revealed heavy deposition of the reaction product in the elastic lamina of the small arteries and arterioles; some dissolution of the elastic fibers was also evident. Pinocytic uptake of the alpha 1-antitrypsin-elastase complex was observed on the abluminal surface of endothelial cells and in smooth-muscle cells bordering the elastic lamina of arterioles. The endothelial cells of the arteries and arterioles retained their normal morphological appearance, although local desquamation was observed in some animals. The results indicate that, when the alpha 1-antitrypsin-elastase complex is present in the circulating blood, it is incorporated into the elastic lamina through the endothelial layer. This results in liquefaction of the lamina, desquamation of endothelial cells and leakage of the complex into the perivascular tissues via the vascular walls. However, some of the complex seems to be excreted very quickly from the kidneys.