High-grade gliomas are one of the most aggressive human tumors with <1% of patients surviving 5 years after surgery. Immunotherapy could offer a possibility to eradicate remnant tumor cells after conventional therapy. Experimental immunotherapy can induce partial cure of established intracerebral tumors in several rodent models. One reason for the limited therapeutic effects could be immunosuppression induced by both the growing tumor and the induced immune reaction. NO has been implicated in tumor-derived immune suppression in tumor-bearing hosts, and unspecific inhibitors of NO synthase have been shown to boost antitumor immunity. In this study, we show that the inducible NO synthase (iNOS)-specific inhibitor mercaptoethylguanidine (MEG) superiorly enhanced lymphocyte reactivity after polyclonal stimulation compared with the iNOS-specific inhibitor L-NIL and the unspecific NO synthase inhibitor L-NAME. Both iNOS inhibitors increased the number and proliferation of T cells but not of B cells. When combined during postimmunization with IFN-gamma-secreting N32 rat glioma cells of rats harboring intracerebral tumors, only MEG increased the cure rate. However, this was only achieved when MEG was administered after immunizations. These findings implicate that NO has both enhancing and suppressive effects after active immunotherapy.