It has been shown previously that dietary fat type influences body fat accumulation in rats. The effects of dietary fat type on serum thyroid hormone, activity of Na+,K(+)-ATPase and lipoprotein lipase were studied. Rats were fed an experimental diet containing lard, high oleic safflower oil, safflower oil or linseed oil for 12 wk. Carcass fat content was significantly higher in rats fed the lard diet than in those fed the other diets. However, intra-abdominal adipose tissue weights were not affected by type of dietary fat. The serum triiodothyronine concentration and the activity of Na+,K(+)-ATPase in the liver and skeletal muscle were significantly lower in the lard diet group than in the other diet groups. The lipoprotein lipase activity of abdominal subcutaneous fat was significantly higher in rats fed the lard diet than in rats fed the other diets, but the activity of lipoprotein lipase in intra-abdominal fat was not significantly different. These results suggest that the intake of lard, compared with the intake of the vegetable oils, may decrease Na+,K(+)-ATPase activity in the liver and skeletal muscle by lowering serum triiodothyronine concentration, resulting in the promotion of body fat accumulation.