The effects of hypothyroidism on lipid peroxidation (LP), reactive oxygen species (ROS), and nitric oxide synthase (NOS), levels and expression, in rat brain were examined. Hypothyroidism was induced by administering methimazole in drinking water (60 mg/kg/day). In striatum, motor cortex and cerebellum of hypothyroid rats LP was not modified, whereas LP and ROS increased in amygdala and hippocampus of hypothyroid rats at the third week of treatment with methimazole as compared to euthyroid group values. Regarding NOS participation, only hippocampal constitutive-NOS activity was increased, accompanied by an augmentation in nNOS expression. Results show that hypothyroidism induces selective oxidative stress in both the hippocampus and amygdala, where the nitrergic system is involved. © 2008 Springer Science+Business Media, LLC.