Tolerance of and immunity to the toxic effects of narcotic drugs are primary and striking characteristics in the development of addiction. An antitoxin or antidotal substance is the recognized mechanism of their production in most diseases admittedly developing these characteristics. I have adopted the hypothesis, therefore, that an antidotal substance is manufactured by the body as a protection against the poisonous effects of narcotic drugs constantly administered. Such a substance, manufactured in the body, being antidotal to morphine, might well possess toxic properties of its own, exactly opposite in manifestation to those possessed by morphine and other opiates. Toxic substances exactly opposite to opiate in their action might readily account for the severe withdrawal signs, parallel in their extent to the extent of opiate insufficiency, and resembling in their characteristics the manifestations of acute poisoning. A hypothetical antidotal toxic substance, manufactured by the body as a protection against the toxic effects of continued administration of an opiate drug, will therefore explain the well-known development of tolerance and immunity in these cases, and will account for the violent physical withdrawal signs. In a word, it will explain the disease fundamentals on a definite physical basis. Such an hypothesis will explain the stages of development of addiction before outlined. In the stage of tolerance the antidotal toxic substance has begun to make its appearance in the body and to protect it against slight narcotic excess, but its manufacture is not sufficiently established to continue longer than necessary to neutralize the narcotic administered. In the stage of beginning addiction, or beginning narcotic-need, its manufacture has become more developed and more constant and proceeds for a longer time after the discontinuance of the narcotic drug. In the stage of fully developed addiction, or absolute narcotic need, the manufacture of the antidotal toxic substance has become practically an established pseudo-physiological body-process, and will continue long after the administration of the narcotic drug for reasons into which I have gone elsewhere. In other words, in narcotic drug addiction some antidotal toxic substance has become the constantly present poison, and the narcotic drug itself has become simply the antidote demanded for its control. In brief, fundamentally and basically, narcotic drug addiction is a condition presenting definite physical phenomena, symptoms, and signs, due to the presence within the body of some autogenous poison requiring narcotic drug for neutralization of it or of its effects.