Fasting adult male rats were given, by orogastric tube, sucrose, glucose, glucose with fructose and water, and the metabolic rate for the ensuing 150-180 min was measured. It was found that there was an increase in metabolic rate after all sugars, greatest after sucrose and least with glucose and with fructose. With the glucose:fructose mixture the metabolic rate was greater then glucose and less than an equimolar load of sucrose. The respiratory quotient after glucose was less than that after fructose, whereas with sucrose it was greater than an equimolar mixture of glucose and fructose.