Smooth muscle cells growing in the primary culture derived from outgrowths of the intimal-medial explants of both rat and human arteries were used. The 72-hr sequential glucose uptake by the cells of both species in culture dishes was enhanced only slightly with time by the addition of insulin to culture medium, and this enhancement was statistically not significant. The glucose conversions to CO2 and lipids by the rat and human cells dissociated for tracer study were not affected significantly during the 2-hr incubation by the insulin addition in vitro. The smooth muscle cells of both species cultured for a week in medium enriched with insulin and then dissociated revealed the significantly increased glucose conversion to lipids, while the increase in the glucose coversion to CO2 was not significant in these cells. Thus, the smooth muscle cells of both rat and human seem to show significant metabolic response to chronic, but not acute, exposure to insulin. Therefore, it is likely that the persistent change in the insulin level may lead to abnormal metabolic state in the artery.