At the present time when the chemical exploration of the insulin molecule has yielded spectacular results (257–259) the physiology of its action is still a matter of speculation. Yet in this field, too, much new knowledge has been gained, mainly by the application of new techniques, such as the use of isotopes and in vitro studies with surviving tissues. It is the purpose of this article to examine the conclusions which may be drawn from the facts as they are known1. The central point of the discussion will be the question of whether a single mode or multiple points of action are required to account for the various effects of insulin; having arrived at an answer in favour, essentially, of a single mechanism of action we shall analyse its possible nature. It is not intended to review the literature completely — a well-nigh impossible task in any case — but to consider it only insofar as it seems relevant to the argument.