Muscular exercise induced by electrical stimulation of femoral nerves in perfused rat hindquarters(5 contractions per sec) in the presence of insulin and glucose effected a rapid increase(c. a. two-fold) in the level of citric acid cycle intermediates. The highest values were found within one minute of stimulation. The tissue concent ratios of lactate, pyruvate and alanine increased rapidly on initiation of exercese. Release of lactate also increased rapidly, whereas that of pyruvate was only moderately elevated. In the course of three minute exercese, the sum of alanine, glutamate and aspartate was only transiently elevated. A fall in creatine-p and ATP in the stimulated muscle was accompanied by increases in tissue level of AMP and release of ammonia into perfusing medium. However, the changes in glutamine were small. It is concluded that the pool of citric acid cycle intermediates is expanded during muscular work due (a) to an elevated level of pyruvate, leading to shifts in the levels of alanine and cycle intermediates vie trans-amination reactions and (b) to stimulation of the purine nucleotide cycle due to elevated AMP, resulting in generation of cycle intermediates and ammonia at the expense of aspartate.