The role of nitric oxide in the immunosuppressive effect of Trypanosoma lewisi on the response to Toxoplasma gondii in WISTAR rats was evaluated. Two groups of rats were infected with T. gondii tachyzoites. One of these groups had been infected with T. lewisi four days earlier. A third group vas infected with T. lewisi only. The concentration of nitrates, as a reflection of nitric oxide production, was measured in serum during 10 days after infection with T. gondii. The results show that rats infected with T. lewisi only, do not at any time display altered levels of serum nitrate. T. lewisi infection does, however, partially inhibit the increase of serum nitrate levels caused by T. gondii 2 days after infection, a time point at which T. gondii multiplication in T. lewisi infected rats is exacerbated.