The remarks put forward in this paper on the comprehensive subject of the action and the nature of enzymes were intended as a guide for a lecture I was prevented from delivering. Now that I am no longer in a position to participate in experimental inquiry I feel inclined to publish them as a concise, and, it must be added, as a very incomplete compendium of the conceptions I derived from the work of others and partly also from my personal investigations. Already in the very first stage of the knowledge of chemical changes in the animal body, observers were struck with the remarkable phenomenon that various conversions are brought about much more easily in the body than out of it. It became conspicuous especially with regard to the food taken into the alimentary canal. Solid food, such as meat e. g.. when reappearing from the stomach a short time after it had been taken up by it, proved to have been changed into a thin pap, just as happens outside the body through prolonged boiling in water. Initially the body temperature was suspected to be answerable for it.