The kinetics of the mitochondrial respiratory chain of intact muscle and the concomitant changes of the intercellular pH were investigated. Addition of lactate and pyruvate under resting conditions produces reductions of DPN and cytochrome b, and, occasionally, of cytochrome c and flavoprotein. Succinate gives similar but smaller changes. In recently excised muscles moderate contractile activity produces a reduction of cytochrome c and oxidations of DPNH, cytochrome b, and sometimes of the flavoproteins. Tetanic contractions and larger numbers of twitches produce reductions of DPN and of cytochromes b and c. In sartorii of the tropical toad, stored for approximately 2 days at 0-3 degrees C, contractile activity always gives rise to long lasting oxidations of DPNH and cytochrome b. Addition of pyruvate or lactate shortens these oxidation cycles with a concomitant reduction of cytochrome c. These responses to contractions agree with those of mitochondria isolated from leg muscles of the toad upon the addition of ADP. Apparently the mitochondria in resting, excised muscles are not supplied with an excess of substrate. Measurements on the intercellular pH showed that even limited activity ( < 5 twitches) initiates glycolysis. The primary control of respiration resides, nevertheless, in the ADP concentration, rather than in the levels of substrate or inorganic phosphate. The results are quantitatively consistent with the view that ATP is the primary energy donor for muscular contraction.