Experiments on cats 53 Subcutaneous injection 53 Injection into the peritoneal cavity 56 Administration by mouth 57 Summary 59 Experiments on dogs 60 Administration by mouth 60 Subcutaneous injection 60 Experiments on puppies 61 Summary 62 Chronic caffein intoxication 63 Experiments on rabbits 63 Experiments on dogs 75 Discussion of results 91 General summary and conclusions 95 Bibliography 97 THE TOXICITY OF CAFFEIN. INTRODUCTION. Comparative physiology has established the fundamental fact that some properties are common to all forms of living matter. But the same method of inquiry has also led to the recognition of marked differences in the physiological processes of various species of animals. Among the most important investigations which contributed to the knowledge of such variation of function are the studies in comparative metabolism. It is now recognized that metabolism is in some respects quite different in herbivora and in carnivora. Some forms of oxidation are much greater in the rabbit than in cats and dogs. Nuclein metabolism presents important differences in the rabbit and in man, while the mode of neutralizing acid in the body may be cited as another variation in the metabolism of these forms. Perhaps the most striking examples of differences in the metabolism of different organisms is furnished by the results of studies on the fate of certain poisons introduced into the body.