Pathological effects of the chemotherapeutic agent 5-FU on mice testes and their sperm morphology were studied. Animal were injected intraperitoneal with small dose of 5-FU (5, 10 and 15 mg/kg) given for five consecutive days. Treated mice were left for 35 days before taking the histological and sperm samples. Immature germ cells sloughing, spermatid haloapperance and multinucleated giant cells were detected in the testes of the treated mice. All used 5-FU doses (5, 10 and 15 mg/kg b.wt) significantly (p<0.05) increased the numbers of morphologically abnormal sperms compared with the control. However, there were no significant statistical differences between the three doses of 5-FU. Also, it was found that the mean values of the morphologically abnormal sperms increased as the drug doses increased.