Abnormal growth of vascular smooth muscle cells (SMC) is frequently associated with hypertension and atherosclerosis, and homeostasis within a normal vessel is maintained by the balanced generation of both vasoconstrictors and vasodilators. Moreover, several endogenous vasoconstricting factors induce SMC growth, whereas several vasodilators inhibit SMC growth. Inasmuch as adenosine is a potent vasodilator, it is possible that it too could inhibit SMC growth. Hence, the effects of adenosine (10⁻⁸ to 10⁻³ mol/L), 2-chloroadenosine (a stable analogue of adenosine; 10⁻⁸ to 10⁻³ mol/L), and 8-bromo-cAMP (10⁻⁸ to 10⁻³ mol/L) on fetal calf serum (FCS; 2.5%)–induced growth of rat aortic SMC were evaluated. Growth was analyzed by assaying DNA synthesis (thymidine incorporation in SMC pulsed for 4 hours with 1 μCi/mL [³H]thymidine) and cell proliferation (change in cell number). Growth-arrested SMC were treated with 2.5% FCS in the presence and absence of adenosine, 2-chloroadenosine, or 8-bromo-cAMP for 24 hours for DNA synthesis or 4 days for cell proliferation. All three substances inhibited DNA synthesis and cell proliferation in a concentration-dependent manner. Compared with adenosine, 2-chloroadenosine was more potent in inhibiting growth. The inhibitory effects of 2-chloroadenosine were reversed by KF17837 (a specific A₂ receptor antagonist) but not by DPCPX (a specific A₁ receptor antagonist). Furthermore, the inhibitory effects of 2-chloroadenosine were not mimicked by CGS21680 (an A_2a receptor agonist), and the effects of N⁶-cyclopentyladenosine (CPA; an A₁ receptor agonist) were not markedly more potent than those of 2-chloroadenosine, suggesting that the inhibitory effects of adenosine are possibly mediated via A_2b receptors. These studies provide evidence that adenosine inhibits SMC growth and suggest that a decrease in local levels of adenosine may initiate SMC growth and contribute to the vascular remodeling process observed in hypertension and atherosclerosis.