Enzymatic activities of glutathione peroxidase, glutathione-S-transferase, glutathione reductase and catalase, as we 11 as the glutathione content were measured in the brain tissue of regularly cycling rats at dioestrus, proestrus an*d estrus. The activity of glutathione peroxidase was found to be suppressed at proestrus, whereas that of catalase was increased at dioestrus. Glutathione transferase and glutathione reductase activities, as well as the glutathione comtent appeared to be stable during the oestrous cycle. These results suggest that, in the female rat, glutathione peroxidase and catalase activities in the brain tissue are influenced by the ovarian hormone status.