Methyl tertiary-butyl ether [MTBE] is used to reduce carbon monoxide and ozone in urban air and to boost fuel octane. There is a lack of knowledge in the literature about the histomorphometric changes of the testis following exposure to MTBE. Therefore, this experimental study was performed to determine the effect of oral exposure to MTBE on histologic and histomorphometric changes of testis in adult rat. A total of 25 adult male Sprague-Dawley rats were randomly divided into five equal experimental groups: control, almond oil and three treatment groups which received 400, 800 and 1600 mg/kg/day MTBE in almond oil by gavages for 30 consecutive days. Histomorphometric analysis showed no significant difference in absolute and relative testis weight, connective tissue thickness, germinal epithelium height, tunica albuginea thickness and Sertoli cell numbers between experimental groups [P>0.05] . However, trend analysis showed that the seminiferous tubule diameter increased and interstitial cell numbers as well as spermatocyte and spermatid cell numbers decreased significantly in MTBE treated groups [P<0.05] . It may be concluded that MTBE could exert adverse effects on spermatogenic cells in adult rat. Whether the observed changes in the present study are due to the direct effect of MTBE via passing blood-testis barrier or its indirect effect through another mechanism should be elucidated in future studies.