Objective: In this study, it is aimed to constitute a chronic toxoplasmosis model using toxoplasma gondii (T. gondii) ME49 strain that is supposed to produce tissue cysts, to compare healthy and infected mice in terms of behavioral changes. In addition, the relationship between behavioral changes and brain lesions was questioned. Method: Before experimental application, out of a total of twenty-one 2-month- old Swiss albino mice, T. gondiantibody-free, 14 were infected by intraperitoneal (IP) (n=8) or oral (n=6) inoculation of 2x102 Toxoplasma gondii ME49 oocysts. Four months later, fear- and anxiety-related behavioral changes in infected and healthy control groups were comparatively evaluated with plus-maze test. At the end of the experiment (45 day after inoculation), euthanasia was carried out on all mice and their brains were examined histopathologically and immunohistochemically for the presence of T.gondii tissue cysts. Results: Infected mice had low levels of anxiety, they have enteredthe open arms more frequently and have spent more time in the open arms of the maze compared to controls. Pathologically, in the infected group, high rates of gliosis, perivascular cell infiltration, meningitis, and neuron necrosis were observed especially in the parietal and temporal lobes, cornu ammonis, amygdala, and thalamus compared to the other parts of the brain. A higher number of tissue cyst formations were positively correlated with the lesion severity in these parts of the brain. Conclusion: It can be suggested that, tissue cysts and neuropathological changes in chronically infected mice brains have direct concern with behavioral manipulation which results from low anxiety levels. (Archives of Neuropsychiatry 2012; 49:139-144) Key words: Toxoplasma gondii, mice, tissue cyst, behavior Amac: Bu calismada; kist olusturan T. gondii ME 49 susu ile enfekte farelerde olusan anksiyete ile iliskili davranis degisiklikleri ve beyin lezyonlariyla iliskisinin arastirilmasi amaclanmistir. Yontem: Uygulama oncesi serumlarinda anti-T. gondii antikorlarinin olmadigi gosterilen 2 aylik 21 adet erkek Swiss albino fareden deney grubundaki 14'une; intraperitoneal (IP) (n=8) ve oral (n=6) yollarla 2x102 Toxoplasma gondii ME49 ookisti verildikten 4 hafta sonra, anksiyete veya korku ile iliskili davranislari T. gondii ile enfekte fareler ile kontrol grubu saglikli farelerde karsilastirmali olarak degerlendirildi. Deney bitiminde (inokulasyon sonrasi 45'inci gun) farelere otenazi yapilarak, beyinleri histopatolojik olarak ve T. gondiidoku kisti antijeni yonunden immunoperoksidaz test ile incelendi. Bulgular: T. gondii ile enfekte farelerin daha dusuk duzeyde anksiyeteye sahip olduklarini, enfekte olmayan kontrol grubundaki farelere gore daha yuksek oran ve sureyle acik kolu tercih ettiklerini ortaya koydu. Patolojik olarak; enfekte grupta ozellikle parietal ve temporal loblar, kornu ammonis, amigdala, talamus ve cevresinde, beynin diger bolumlerine oranla daha yuksek derecede gliozis, perivaskuler hucre infiltrasyonu, meningitis ve noron nekrozlari gozlendi. Beyinde T. gondii doku kisti yerlesimi de anilan lezyon siddeti ile dogru orantili olarak diger alanlara oranla daha yuksek sayidaydi. Sonuc: Kronik toksoplazmozlu farelerin beyinlerinde sekillenen T.gondiidoku kistleri ve noropatolojik degisikliklerin dogrudan anksiyetenin azaltilmasina yonelik davranis manipulasyonu ile iliskili oldugu one surulebilir. (Noropsikiyatri Arsivi 2012; 49:139-144) Anahtar kelim eler: Toxoplasma gondii, fare, doku kisti, davranis