Genetically obese (fa/fa) Zucker rats maintain a normal lean body size but deposit excessive amounts of body fat. Preferential use of substrates for lipid synthesis results in inefficient use of dietary nitrogen for protein deposition. Three studies were carried out to determine whether an increased protein requirement caused hyperphagia and whether young Zucker rats preferentially regulated protein or energy intake. Rats were offered isoenergetic diets with nitrogen contents ranging from 4.5 to 53.2 mg/g, or isonitrogenous diets with energy contents of 2.2, 3.3 or 4.2 kcal/g. In both situations obese rats had significantly higher food intakes than lean rats. Within phenotype the rats maintained an almost constant energy intake so that nitrogen intake was proportional to dietary nitrogen concentration. In a third experiment lean and obese rats were given different proportions of their control protein intake by stomach tube. Energy intake was determined by voluntary consumption of a protein-free diet. Within phenotype energy intake was the same for all levels of protein intake. It appears that obese Zucker rats regulate energy intake at an elevated level and that protein intake is determined by dietary nitrogen content. Hyperphagia does not appear to result from a desire to obtain protein.