Toxoplasma gondii infection has a worldwide distribution. Pyrimethamine (PYR) is the most effective drug for treatment of toxoplasmosis. Unfortunately, it has low oral bioavailability which requires an increase in the dose that results in increased its side effects. Nanostructures showed promising potential to overcome this problem. This study aimed to evaluate the effect of intraperitoneal injection (IP) of PYR-loaded niosomes compared to PYR in the treatment of acute toxoplasmosis in experimentally infected mice. The study employed 240 mice that were divided into groups. Group I included Ia (20 uninfected untreated), Ib (20 infected untreated), Ic (non-infected and injected with placebo niosomes) and Id (20 non-infected mice injected with dimethyl sulfoxide). Groups II and III (a & b/each, 40 mice/each) were treated with PYR and PYR-loaded niosomes respectively in doses of 5 or 10 mg/kg/day for four successive days. Then all mice were sacrificed and their peritoneal fluids were examined by the scanning electron microscopy. Livers, spleens and brains were used for parasite count and for histopathological examination. This study showed that the niosomes improved the efficacy of PYR in the treatment of acute toxoplasmosis in mice. It was evidenced by increased the survival rate, decreased tachyzoites count, morphological changes of the tachyzoites and decreased inflammation. Niosomal PYR was effective at low dose with its efficacy being even greater than that of the solution even at high dose. It was concluded that PYR-niosomes formulation is a powerful alternative for reduction of PYR dose and its side effects. K e y w o r d sToxoplasma gondii.