The objective was to evaluate the mitotic index of root meristems of Allium cepa, exposed to different concentrations of methotrexate. Study with quantitative and experimental approach. Onion bulbs were used, whose water was changed every 24 hours during a period of 7 days prior to the analysis, in order to induce the growth of the root meristem, establishing two types: the control group, that were the bulbs that continued with water, and the experimental group, consisting of the bulbs exposed to different concentrations of methotrexate solution (0.1, 1 and 2.5 mg/mL). Subsequently, the rootlets of both groups were fixed for the study of the mitotic index, determining 3 repetitions for each sample and coloring each unit of analysis with aceto-orcein staining. The data was processed with the SPSS version 21 software, performing a variance analysis (ANOVA) and the Tukey's test at 5% significance. It is concluded that as the concentration of methotrexate is higher, the rate of cell division of Allium cepa "onion" is lower, demonstrating its cytotoxicity. This indicates the inhibitory action of methotrexate on cell division.