The results of this study demonstrate the fact that insulin inhibits the ketogenic action of a crude neutralized alkaline extract of the anterior hypophysis which has been found to be effective only in the presence of the liver. A similar ketolytic effect is obtained with ergotamine tartrate. It is concluded that these substances exert their antiketogenic effects on the liver. As a result of various considerations it is suggested that insulin exerts a ketolytic effect when it is antiglycogenolytic in action and, conversely, it exerts a ketogenic effect when it is glycogenolytic with regard to the liver.