The present experiment was designed to test the effect of a specific caloric hunger on the intake of glucose and saccharin solutions, both of which are preferred to water by animals on an ad lib diet.1 Glucose, however, furnishes calories while saccharin has no nutritive value. Hausmann has shown that when animals drink sugar solutions they reduce their food intake, keeping total caloric intake constant.2 Since the drinking of saccharin solutions did not affect food intake, Hausmann concluded that the animals are not 'deceived' by the taste of saccharin. There is evidence, however, which indicates that hunger influences the reward value of saccharin for rats. Sheffield and Roby show that hungry animals learn to respond to a stimulus-pattern related to the availability of saccharin while satiated animals do not.3 Since saccharin is supposed to have no effect on hunger, we might assume that saccharin acts as a secondary reinforcement because it has taste qualities which are similar to the sugars. If saccharin is a secondary reinforcement, then introduction of hunger should affect responding for sugar and saccharin similarly. This is not the case, however, as shown by a study of the bar-pressing response with rats.4 Increasing both the specific caloric hunger and general hunger decreased the rate of responding for a saccharin reinforcement, whereas the opposite was true for glucose. The purpose of this experiment is to test whether caloric hunger differentially effects the drinking of glucose and saccharin solutions in a free