The neuromodulator adenosine is one of the major endogenous inhibitors of overactive excitatory neurotransmission. Adenosine receptors have been identified on neuronal but also on glial surfaces, indicating a role of glial cells in mediation of adenosine effects. Microglia, the immunocompetent cells of the brain, typically respond with proliferation, migration and production of inflammatory substances to viral or bacterial stimuli or to cell damage and degeneration. Since adenosine is released in large amounts in conditions of, for example, hypoxic or ischemic stress, it might be involved in the activation process of microglia. Proliferation of microglia was determined by incorporation of [3H]thymidine into microglial DNA after stimulation with adenosine A1- and A2-receptor agonists. N6-Cyclopentyl adenosine (CPA) and CGS-21680, a specific adenosine A2-receptor agonist had no effect on microglial proliferation. However, combinations of CPA and CGS-21680 as well as the mixed agonist, N6-ethyl-carboxamido adenosine (NECA) increased incorporation of radiolabel above controls. The effect of NECA was inhibited by the adenosine A1-receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). From these results, it is concluded that proliferation of microglia can be increased only by simultaneous stimulation of both adenosine A1- and A2-receptors. Targeted interference with the activation of A1-adenosine receptors by specific drugs appears to be sufficient to reduce microglial activation. The findings may have implications for the treatment of neurodegenerative diseases in which microglial activation is supposed to play a causative role.