Brazil has a great number of species that present medicinal uses by the local population. Since the toxicity of medicinal plants is a serious public health problem, there is a need for studies on its phytotherapeutic actions. Plants of the genus Banisteriopsis are known both for their medicinal and psychotropic activities. The objective of the present study was to evaluate the cytotoxic and genotoxic effects of methanolic extracts of Banisteriopsis malifolia (Nees & Mart.) B.Gates and Banisteriopsis variabilis B.Gates, on concentrations of 9, 20, 33, and 50, on germination of onion (Allium cepa L.) seeds. Four replicates of 50 onion seeds, moistened with 10 ml of each extract and distilled water used as negative control, were done in a completely randomized design. In both treatments, at concentrations of 33% and 50%, we observed yellowish roots, thicker and less turgid in relation to the control. There was a reduction in cell divisions at the 33 and 50% concentrations and a positive correlation was observed with chromosomal aberrations at the same concentrations. These results suggest the cytotoxic and genotoxic activity of plants B. malifolia and B. variabilis directly proportional to the concentrations tested.