Focal proliferation of smooth muscle cell (SMC) is an integral part of atherosclerotic plaque formation: characterization of regional variation in SMC growth kinetics is therefore important to the understanding of atherogenesis. SMC growth kinetics of rabbit abdominal and thoracic segments were compared. Rabbit aortas were denuded of endothelium and the animals killed after3H-thymidine and Evans blue injections at 0 to 48 days after denudation. Incorporation of3H-thymidine into both aortic segments peaked at 48 hours; no detectable incorporation occurred in the first 24 hours. Abdominal segment DNA specific activity (SA, dpm/μg DNA) and total kinetic activity (TKA, dpm/0.1 mm internal elastic lamina) at 48 hours were significantly greater than values for the thoracic aorta. Abdominal SA and TKA curves decreased exponentially after the 48-hour peak and paralleled thoracic levels after day 7. SA and TKA values for each segment reflected the subsequent SMC intimal growth rates as measured morphometrically. Therefore, both segments share similar growth kinetic charac-teristics; however, the abdominal response to intimal injury is greater than the thoracic and leads to greater myointimal proliferation. The difference in response to injury in the two segments suggests regional variation in SMC'swhich are phenotypically similar.Circ Res 47:182-189, 1980