The study aimed to demostrate the genotoxicity of phenytoin in polychromatic erythrocytes (PCE) of Rattus norvegicus strain Holtzman, though using micronucleus test. Five groups of five specimens each one were established, a negative control group (SSF), three experimental groups treated with phenytoin (5, 7.5 and 10 mg / kg BW), treaments for these groups were intraperitoneally during 14 days; a positive control group treated with cyclophosphamide (50 mg/kg BW) intraperotineally for 24 hours. Micronucleated PCE frequency in a total of 2000 per animal wes determined. The results show significative increase in genotoxcicity rate of phenytoin (4.62 ± 0.54, 5.39 ± 0.43, 5.49 ± 0.41) and cyclophosphamide (8.18 ± 0.29) compared to negative control group (0.08 ± 0.15); significative increase in the frequency of micronucleated EPC in individuals treated with phenytoin (92.4 ± 10.89, 8.52 ...