Aim of the work-Methotrexate (MTX), a structural analogue of folic acid, is an antineoplastic and antirheumatic agent which is used in a variety of clinical schedules and combination therapy regimens in man. Material and methods- Sixty mice of nearly the same age were randomly categorized into four groups (one control and three treated groups with different doses of methotrexate). Mice of the treated groups 1, 2 and 3 were intraperitoneally injected with a single dose of methotrexate (2.5, 5 or 10 mg/kg b. wt. respectively) at the first day of the experiment. All the control and the treated animals were sacrificed after 24, 48 or 72 hour by cervical dislocation post treatment. Results-Methotrexate treatment induced structural and numerical chromosomal aberrations in male mice bone marrow cells which were significantly increased (P< 0.001) by dose and time. Structural aberrations were chromosomal gap, fragment, break, centromeric attenuation, deletion, centric fusion, ring formation, end to end association and beaded chromosomes. Numerical aberration was polyploidy. Also, methotrexate treatment decreased the mitotic index in bone marrow cells of all the treated mice in comparison with the control group by increasing dose and time of treatment. Comet assay results indicated that treatment with methotrexate significantly increased (P< 0.001) DNA damage in the blood leukocytes in dose and time dependent manner. Conclusion- It can be concluded that methotrexate induced genetic damage on the chromosomes and DNA content of male albino mice even after single treatment with low doses.