Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. Antioxidant defenses help to prevent free radical-mediated injury (1, 2). These substances, including enzymatic (catalase, superoxide dismutases, glutathione peroxidase and glutathione reductase) and non-enzymatic [vitamin E, vitamin C, glutathione (GSH) and uric acid] categories, protect an organism from oxygen free radicals (3). Red blood cells (RBCs) are the main source of antioxidants; in vitro and in vivo tests have been performed to demonstrate the importance of RBC antioxidant enzymes in protecting target cells from reactive oxygen species (ROS) (4, 5). Therefore, the amount of antioxidants in RBC is a suitable marker for evaluating chronic ROS exposure. GSH is the most abundant thiol in cells and acts as a major antioxidant in addition to its other biological functions in RBC (6). Catalase is an important antioxidant produced by RBC; thus, catalase levels can indicate ROS levels (7). However, limited information is available on the effects of physiological sex steroid changes during the menstrual cycle on the GSH and catalase in RBC in a population of healthy normomenorrhoic females. Therefore, the present study evaluated the effects of physiological sex steroid changes during the menstrual cycle on GSH and catalase activities. Females with regular menstrual cycles were selected to be part of the sample and the changes in the GSH and catalase levels during the menstrual cycle were measured. The resulting correlation between estrogen and antioxidant systems can be used for analyses and discussion. Hope I have answered your query. Let me know if I can assist you further. Regards, Dr. Rajesh Raghuwanshi