The plant-derived cannabinoids Δ⁹-tetrahydrocannabinol (THC) and cannabidiol (CBD) both have immunosuppressive effects; although some effects of THC are mediated by the CB₂ receptor, CB₂ binds CBD weakly. In examining the effects of THC and CBD on microglial proliferation, we found that these compounds potently inhibit [³H]thymidine incorporation into a murine microglial cell line with no effect on cell cycle. Treatment with THC and CBD decreased [³H]thymidine uptake into microglia, with IC₅₀ values that match inhibition of [³H]thymidine incorporation into DNA. CBD and, less potently, THC decreased uptake of [³H]adenosine to a similar extent as [³H]thymidine in both murine microglia and RAW264.7 macrophages. Binding studies confirm that CBD binds to the equilibrative nucleoside transporter 1 with a K_i < 250 nM. Because adenosine agonists have antiinflammatory effects, and because uptake of adenosine is a primary mechanism of terminating adenosine signaling, we tested the hypothesis that CBD is immunosuppressive because it enhances endogenous adenosine signaling. In vivo treatment with a low dose of CBD decreases TNFα production in lipopolysaccharide-treated mice; this effect is reversed with an A_2A adenosine receptor antagonist and abolished in A_2A receptor knockout mice. These studies demonstrate that CBD has the ability to enhance adenosine signaling through inhibition of uptake and provide a non-cannabinoid receptor mechanism by which CBD can decrease inflammation.