Stage conversion between tachyzoites and bradyzoites of Toxoplasma gondii was investigated in vitro by using murine bone marrow-derived macrophages (BMMs) as host cells. Following infection of untreated BMMs with tachyzoites, spontaneous expression of bradyzoite-specific antigens (Bsa) occurred at low frequency with Toxoplasma strain-dependent ratios from 0.03 to 2%. As previously described for peritoneal macrophages, activation of tachyzoite-infected BMMs with gamma interferon (IFN-gamma) or lipopolysaccharide resulted in the induction of Bsa. When bradyzoites were used for infection, prolonged expression of Bsa could be observed in IFN-gamma-activated BMMs. The induction of Bsa expression seemed to be closely linked to parasite multiplication and increased to maximal values of 50 to 70% in intermediately activated macrophages with nitric oxide (NO) levels that allowed reduced parasite replication. Identical results in stage conversion were obtained when sodium nitroprusside was used as a source of exogenous NO, indicating that NO might be a molecular trigger of stage conversion. NO is reactive with iron-sulfur centers in proteins, thereby inhibiting proteins involved in the mitochondrial respiratory chain. Using oligomycin and antimycin A as inhibitors of mitochondrial function, growth inhibition of parasites and induction of Bsa were obtained. Since microglia are the functional correlates of macrophages in the central nervous system and inhibit T. gondii upon activation with IFN-gamma, a similar mechanism might be involved during cyst development in the brain.