Severe, complicated atherosclerotic lesions of swine abdominal aortas were induced by combination of balloon injury and atherogenic (HC) diet. After a reference group was killed at six months, the remaining animals were fed a mash diet for six weeks, five months, or 14 months. Extensive biochemical studies of lesions and nonlesion areas of aortas from HC diet-fed swine, and nonlesion aortic tissue from mash-fed animals emphasize the following points. Nonlesion tissues from either mash or HC diet-fed animals were remarkably similar in all features studied, and did not change much over a period of at least 20 months. Lesions differed from the adjacent nonlesion tissue in most aspects; the HC nonlesion tissue does not appear to be an intermediate in lesion development. The lesions were dynamic, changing generally in the direction of normalcy, from relatively active to more quiescent. Lesion changes did not appear immediately after institution of the regression regimen; some were delayed several months. When changes did occur, in general they were not linear with time. Whether a complete return to normalcy would result from a longer regression period needs further investigation.