Toxoplasma gondii (T. gondii), an opportunistic protozoan, is an important cause of central nervous system (CNS) infections in immunosuppressed patients. The present study focused on the interaction between T. gondii and microglial cells from the brain of neonatal Balb/c mice. Preincubation of the murine microglial cells with recombinant interferon-gamma (rIFN-gamma) and lipopolysaccharide (LPS) induced significant inhibition of T. gondii replication in a dose dependent manner. This antiparasitic effect in microglial cells was correlated with the induction of the L-arginine-dependent generation of reactive nitrogen intermediates (RNIs). Tumor necrosis factor-alpha (TNF-alpha) was also involved in the toxoplasmastatic activity. Microglial cells incubated with recombinant TNF-alpha in combination with a non-activating concentration of rIFN-gamma released substantial amount of RNIs. Neutralizing antibodies against mouse TNF-alpha inhibited the release of RNI by rIFN-gamma activated macrophages. In summary, the present results show that activation of microglial cells by rIFN-gamma and LPS induce the production of nitric oxide (NO) by these cells via an L-arginine dependent pathway. NO appears to be the effector molecule mediating the toxoplasmastatic effects in these cells.