Four rats were fed with liver only (liver rats) and four rats were fed with beef only (beef rats) during the first three months of their lives. Thus, they were deprived of a variety of food tastes. The rats were then trained in instrumental conditioning. After finishing the training with the liver reward, the liver rats were not able to extinguish the response, however, after achieving a criterion performance with the beef reward, the extinction of the response was very easy in those rats. The opposite data were obtained in the beef rats. These results show that liver was more palatable than beef for deprived rats. In contrast, there was no difference in palatability between liver and beef in another four rats non-deprived of a variety of food tastes.