In the present study, genotoxic effect of methyl tert butyl ether MTBE was analyzed by measuring chromosomal aberrations (CAs) in bone marrow cells of rats. Rats administered MTBE orally at 800, 1600mg/kg/day in corn oil for 14 and 28 consecutive days. Control rats received injection of distilled water. An additional two groups of rats received corn oil and served as vehicle controls. Treatment of corn oil for 14 and 28 days failed to induce chromosomal aberrations. The highest percentage of chromosomal aberrations was produced by the two tested dose 14 days after treatment. The most structural aberrations were Robertsonion translocations, deletion, dicentric, end to end association while, ring, acentric fragment and gaps were rare. The present results indicate that MTBE is harmful to mammalian genetic material.