The metabolic costs of rough-and-tumble play behavior were studied in juvenile rats. Using indirect calorimetry, it was determined that energy expenditure during play is increased by 66-104% over the resting metabolic rate, indicating that play accounts for between 2% and 3% of the total daily energy budget of the rat. In a subsequent experiment, food intake and body weight were monitored for 3 weeks in rats allowed to play for 1 hr/day and in rats not allowed to play. While the body weights of the two groups did not differ significantly from each other, those rats allowed to play ate 7% more over the 3-week period than did those rats not given an opportunity to play. These data are consistent with previous reports describing the energetic costs of mammalian play, with play accounting for less than 10% of the daily energy budget in three species tested so far. These data are also consistent with viewing play as a type of exercise and may lead to a better understanding of putative benefits of this behavior.