Proliferation of smooth muscle cells (SMCs) is a major event in vascular development and atheromatous plaque formation. In order to characterize SMC replicative potential, newborn rats have been injected with 3H-thymidine (3H-TdR) during their first week of life when most of SMCs were proliferating; then, 3H-TdR labelling was evaluated in adult rats. Depending on the number of mitosis that SMCs had accomplished after the first week of life, four different SMC subpopulations could be defined indicating that rat aortic SMCs are heterogeneous in their replicative activity. 5-Bromo-2'-deoxyuridine (BrdU) incorporation after balloon induced endothelial denudation of rat aorta in adult rats showed that SMCs entering into the cell cycle were mainly devoid of 3H-TdR labelling. This category could derive from two distinct SMC subpopulations: SMCs which were arrested in their proliferation before or just after birth or SMCs which had actively replicated during development. Thus, one (or two) subpopulation(s) of rat aortic SMCs, characterized by a particular replicative activity during development, is (are) selectively activated after balloon induced endothelial denudation.