Several studies support an association between gonadal hormones and oxidative state. This study aimed to determine the consequence of the absence of ovarian hormones on the oxidative status of animals submitted to acute stress induced by CO(2) inhalation. We also evaluated the effect of pregnanolone administration upon the oxidative status in distinct brain structures of ovariectomized (OVX) rats exposed to CO(2). Female rats were divided into intact and OVX and exposed or unexposed to CO(2). Oxidative status was evaluated by 2',7'-dichlorofluorescein (DCF) assay, assessment of malondialdehyde (MDA), as an indicator of lipoperoxidation (through the thiobarbituric acid-reactive substances assay, TBARS), and the total antioxidant reactivity (TAR). Both DCF and TBARS were increased in the hypothalamus of animals submitted to OVX and stress. Nevertheless, free radical production and MDA levels were not affected in either condition alone. In the cerebral cortex, lower MDA levels were observed in OVX animals. Pregnanolone administered to rats submitted to CO(2)+OVX resulted in reduced MDA levels and free radicals production in hypothalamus. We suggest that ovarian hormones may protect the hypothalamus against oxidative stress, particularly when the animals are submitted to challenges. Pregnanolone may protect, at least in part, the hypothalamus of OVX rats from oxidative stress.