The effects of six balanced diets for 3 weeks on dietary intake, growth, liver weight and fat, plasma cholesterol, total antioxidant capacity, liver glutathione status and antioxidant enzymes in growing male Wistar rats were studied. Ten rats per group were fed casein- and soyabean-based diets with or without 2·4% cholesterol-raising agent. Seven percent of the diet consisted of a cellulose–wheat starch mix (35:65; control diets), freeze-dried nori (nori diets) or konbu (konbu diets). The 7% dietary supplement of seaweeds was well accepted and induced normal growth rates in rats. Except for food intake, total and reduced glutathione and total antioxidant capacity, dietary cholesterol addition significantly affected (at least P<0·05) all parameters studied. Alga consumption affected total and reduced glutathione, glutathione reductase activity, plasma cholesterol, and total and cholesterol-adjusted total antioxidant capacity (at least P<0·05). A significant cholesterol–alga interaction was found for liver weight, total glutathione peroxidase (GSH-Px) and the Se-dependent GSH-Px:total GSH-Px ratio (at least P<0·05). GSH-Px activity increased in cholesterol-fed nori rats mainly as Se-dependent GSH-Px, while in konbu and control groups the GSH-Px activity was related to increases in both non-Se-dependent and Se-dependent GSH-Px activities. The decrease in the antioxidant status of konbu rats was related to the high As content of this alga, which led to a compensatory increase in glutathione reductase activity in these animals. In conclusion, although some antioxidant compounds are present in algae, other dietary compounds, such as As, induced poor antioxidant status in rats.