In order to determine the anti-atherosclerotic action of elastase, the aortas from four groups of rats--group N fed a normal diet, group D fed an atherogenic diet with vitamin D2, group N + Ela fed a normal diet with elastase, and group D + Ela fed an atherogenic diet and vitamin D2 with elastase--were studied histologically after 3, 6, 9, and 12 weeks of feeding. Group D displayed a remarkable atheromatous change, while the change was insignificant in group D + Ela. The histopathological findings in group D consisted of marked rarefaction, fat deposition, and atheroma in the subendothelial tissue and tunica intima; Van Gieson's staining revealed a conspicuous decrease in elastic fibres, and the remaining elastic fibres had such abnormalities as a tortuous course, bends, and fissures. For group D + Ela, on the other hand, there were few foam cells in the subendothelial tissue and tunica intima and much less marked rarefaction of interstice and fat deposition. Further, the arrangement of elastic fibres was only slightly disordered, and the decreases in elastic fibres and their fissures were moderate. In sharp contrast to group D. which showed marked calcium deposition, group D + Ela was found to have low calcium deposition. No macroscopic and histopathologic abnormalities were found in groups N and N + Ela. Elastase had an inhibitory effect on experimental atherosclerosis in the rat and also regulated the degradation and biosynthesis of elastin, which resulted in the regeneration of elastic fibres.