The cultured smooth muscle cell offers several advantages to investigation of cell character. It can be utilized to make studies on aging and atherosclerosis. This work was carried out to investigate the effects of intracellular lipid peroxides and aging on the growth rate of cultured aortic medial smooth muscle cells obtained from normal rats and rats with streptozotosin-induced rats. Cells from young and old rats showed similar growth and contained the same amount of intracellular lipid peroxides. However the rate of growth of cells from diabetic rats was inversely proportional to the period of illness and also to the quantity of lipid peroxides contained in the cells. When normal cells were cultured in an adriamycin-containing medium, their growth rate was inversely proportional to the adriamycin concentration of the medium and also the amount of intracellular lipid peroxides.