Measurements have been made of the volume changes accompanying single isometric and isotonic twitches of frog sartorius muscle. The volume change consists of a rapid increase, a subsequent decrease, and a return to the initial volume; the order of magnitude of increase and decrease is 10-5 cc/g of muscle. This volume change is length-dependent: the initial increase becomes more pronounced as the initial length of the muscle is decreased, while the volume decrease is greatest at reference length and is diminished for longer and shorter initial lengths. Muscle volume changes are also dependent upon temperature and amount of shortening: the return phase is prolonged as the temperature is lowered; and, in an isotonic twitch, a volume increase accompanying muscle shortening is superimposed upon the volume change described for an isometric twitch. These "shortening volume changes" may correspond to the volume decrease observed in frog muscle under a passive stretch. If the active state is prolonged by the use of a frog Ringer solution in which iodide ions have been substituted for chloride ions, the time course of the volume decrease is likewise prolonged; this suggests a relationship between the volume decrease and the active state of the muscle.