While Mentha x piperita L. is a popular herb used in the northern region of Portugal to prepare tisanes the information about its possible toxicity is scarce. This Study was designed to assess the toxicity of the aqueous extract on the physiology, morphology and biochemistry of Tetrahymena pyriformis. Mint extract affected the generation time and growth in a dose-dependent manner. Also morphometric features of the cells of T. pyriformis are affected. The MTT assay showed a dose-dependent response of mint extracts.