Insulin affects many processes in muscle and it is difficultto envisage ways in which all these effects can result from a single primary action. Nonetheless it is of interest to speculate how the different actions of the hormone could be linked to each other. The possibility that insulin might induce a conformational change in responsive tissues is considered and how such need not include nuclear events. An alter native suggestion would be for insulin to induce a small rise in intra cellular pH. Such would favour many pH-sensitive reactions and is indicated by studies of the distribution in muscle of DMO and CO<sub>2</sub>/bicarbonate. These results direct attention towards functions of the mitochondria, and a possible explanation of why respiration is not affected by insulin despite changing anabolic load is considered. Mitochondria are very active in translocating ions. Possible ways in which movements of ions in response to pH could affect metabolism are put forward.