Free radicals generated in vivo and peroxidation of the unsaturated bonds of membrane lipids increases fragility and cellular lysis of red blood cells. The essential oils of Piper aduncum and Cinnamomum zeylanicum present antioxidant activity and are popular in some countries to treat gastrointestinal diseases and anxiolytic symptoms respectively. Osmotic fragility assays and morphometric analyses have been used to assess cytotoxic effects and to verify the interaction of compounds present in the essential oils using the membranes of red blood cells (RBC). The aim of this work was to evaluate the effects of the essential oils of P. aduncum and C. zeylanicum on osmotic fragility using osmotic stress and morphology of RBC.