Laboratory bred female mice were used to study the effect of chronic toxoplasmosis on reproductive performance. The animals were infected with small dose (3 x 10(3)) tachyzoites of Toxoplasma parasites. The female mice were mixed with normal males for one week, then separated to monitor their reproductive performance one and two months post infection (p.i). Mice bred one month p.i. showed reproductive failure with one of twenty females delivered two sick newborns. The others did not complete pregnancy and faetal wastage occurred. Mice bred two months p.i. were infertile. Histopathological examination of the ovaries revealed impaired folliculogenesis and atropic degeneration. Coronal sections of cerebrum showed widespread vasculitis, focal disruption of the ependymal cells lining both the lateral and third ventricles. The supraoptic and paraventricular hypothalamic nuclei were deformed and showed pyknotic neurons. Immunoperoxidase staining was used to detect IgG and IgM deposits in brain tissue. IgG deposits were found on the vicinity of Toxoplasma cysts and focally in the paraventricular zone. So, the reproductive failure of infected mice was due to hypogonadotrophic hypogonadism secondary to hypothalamic dysfunction as a result of chronic toxoplasmosis.