Single doses of 40, 80 and 160 mg of cyclophosphamide per kg body weight were administered intraperitoneally to C57BL/6 male mice. Cytogenetic analysis of spermatocytes at diakinesis/metaphase I, influenced as spermatogonia A and B and pachytene spermatocytes, showed the chromosomal damage. Cyclophosphamide induced structural chromosomal aberrations including translocations and interfered with the normal development of bivalents. The dose of 80 mg/kg yielded 0.6% spermatocytes with translocations and 3% translocation carriers in F1 males. Robertsonian translocations were detected in 3 males.