Studies of the metabolic responses of rats to 0.2% dietary theophylline were conducted. Young male rats were fed theophylline‐containing diets or control diets for 8 wk and killed after 22 hr without food. Levels of serum lipids, glucose, and ketones were determined as were hepatic glycogen, lipid, calcium, magnesium, and zinc levels. Epididymal fat pad lipid content, as well as kidney cation levels and hepatic mitochondrial respiration, was also determined. Chronic theophylline feeding lowered blood glucose levels, serum triglyceride levels, hepatic lipid and calcium levels, and the state III respiration with succinate as the substrate. Kidney calcium and magnesium levels were elevated in theophylline‐fed rats as were the serum cholesterol levels. The relationship of these metabolic responses to theophylline feeding is discussed as a function of the theophylline effect on tissue cation levels.