Aspirin reduces the growth of medial and neointimal thickenings in balloon-injured rat carotid arteries. We analyzed the effect of Aspirin on the growth of experimentally induced vascular thickenings in rat carotid arteries. Vascular thickenings were induced by denudation of the endothelium in the left carotid artery with a balloon catheter. Administration of Aspirin-rich food (17.4 g/kg body wt/day) was started 1 week before and continued 2 weeks after injury. Nine rats were used. A control group of equal size received normal food. Sizes of the tunica media, the neointima, and the open vessel lumen were measured on cross sections of carotid segments with the aid of a videomorphometry system. The results show that in the Aspirin group, neointimal lesions are significantly smaller than in the control group (0.14 mm2 versus 0.23 mm2; p less than 0.5). Thickenings of the tunica media are also reduced (0.11 mm2 versus 0.12 mm2; p less than 0.5). It is suggested that Aspirin reduces both medial hypertrophy and neointimal outgrowth in injury-induced atherosclerosis. 