Among different antigens, it has been demonstrated that T. gondii tachyzoites ESAs are highly immunogen and in different studies depending on a mouse model, used adjuvant, method of antigen administration, intraperitoneal dose of injected antigen, and method of antigen preparation, different and sometimes conflicting results have been reported [6, 12, 14]. ESAs are released by three secretory organelles including micronemes (MIC), rhoptries (ROP), and dense granules (GRA). Host cell invasion is mediated by the sequential secretion of the content of all three organelles [20]. The micronemal proteins are thought to function for the host cell recognition and attachment [21, 22]. The rhoptry proteins may function in the formation of the parasitophorous vacuole [20]. The dense granular proteins are released both during and after invasion into the parasitophorous vacuole. They are thought to function for intracellular survival and replication [23]. Several studies have been done on the immunogenicity of T .gondii ESAs, but none of them have focused on aspect of tissues tropism of parasites. Regarding the movement of the parasite in the host body, causing complications such as encephalitis, chorioretinitis, etc., it seems that in addition to evaluate the immunogenicity of vaccines, study of the movement trend of the parasite and parasitic load in different tissues is also essential. Such data are needed to better understand the pathogenesis of toxoplasmosis, and also baseline data for the experimental evaluation of novel vaccines and chemotherapeutics. Thus, according to our previous studies conducted in the field of T. gondii ESAs and their use in immunization studies, in this survey, we monitored and quantified for the first time, the ... ... middle of paper ... ...in was due to immune responses in through of the mouse body, not in brain tissue. In spleen, in addition to above mentioned mechanism, due to filtration of a large number of parasites in the blood stream, trapped in this organ; so compared with some tissues, parasite load was high. Aigner et al. (2010) in a study on 28 positive serum chickens showed that using Real-time Q-PCR, 24 (93%) samples of heart and brain tissues had Toxoplasma DNA and parasite load in brain and heart was 742, 982 parasites per gram tissue, respectively. ESAs in most tissues of immunized mice (even brain) compared to control mice has decreased parasite load. Therefore these antigens with stimulation of cellular and humoral responses and reduction of parasite load in different tissues of host could be evaluable candidate for the development of immunization strategies against toxoplasmosis[27].