The ATP synthase capacity of rat heart myocytes can be measured in sonicates of cultured cardiomyocytes. In these cells, transitions in ATP synthase capacity occur on changing to the anoxic or uncoupled state (drop in ATP synthase capacity of over 40%) or on electrically stimulating the cells to contract (rise of 70%). These changes occur rapidly (half time less than 1 min) and are completely reversed on returning to the original conditions. It is proposed that mitochondria in vivo are directly regulated at the level of the ATP synthase. The naturally occurring inhibitor protein from mitochondria may be responsible for this regulation.