The purpose of this study was to evaluate the effects of Ulva rigida C. Agardh, one of the green algae, on and antioxidative system in the propylthiouracil (PTU)-induced hypothyroid rats. Thirty-two rats randomly divided into four groups: control (C), control+U. rigida extract (C+UR), hypothyroid (H) and hypothyroid+U. rigida extract (H+UR). U. rigida (2%) was administered in drinking water for 5 weeks after the induction of hypothyroidism. Hypothyroid rats were under oxidative stress as reflected by icreased plasma and tissue malondialdehyde (MDA) levels. U. rigida reduced serum total cholesterol and,- triglyceride levels and plasma and heart skeletal muscle, liver and,- kidney tissue MDA levels in the H+UR group. Serum total cholesterol and tissues MDA levels were reduced in the C+UR group. Whole blood glutathione peroxidase and erythrocyte superoxide dismutase activities were increased in the H+UR and C+UR groups compared with those of te respective control groups. Paraoxonase and arylesterase activities were lower in the H group and U. rigida increased paraoxonase and arylesterase activities in C+UR and H+UR groups. We conclude that hypothyroidism is associated with oxidative stress and, U. rigida extract might have a potential use as a protective antioxidant agent in hypothroidism.