Histochemical examination of the middle ear muscles of cats and rabbits was carried out to investigate the difference among muscle fibers composing them and the correlation between the function and the structure of individual muscle fibers.<BR>The muscle was frozen and sectioned about 25 -30μ thick. For the histochemical demonstration of lipid, succinic dehydrogenase, cytochrome oxidase, and DPN-diaphorase, the methods established by Lison, Wachstein et Meisel, Gräff, and Farber et al were applied respectively.<BR>The results obtained were as follows.<BR>(1) It was revealed that the tensor tympani and stapedius of cats and rabbits are composea of the fiber giving a high enzymatic activity of succinic dehydrogenase, cytochrome oxidase and DPN-diaphorase and showing an intense affinity to Sudan black B and the fiber giving very slight reaction of them. The former may correspond to red or dark fiber and the latter to w hite or light fiber. Besides these, there exists a third type of muscle fiber in the stapedius, which shows a moderate enzymatic activity and is intermediate between the red and white muscle fibers.<BR>(2) Generally, the white fiber showed a tendency to be thick, while the red fiber to be thin. In the tensor tympani the distribution ratio of the red and white fibers, which markedly varied individually, was approximately 3: 2 or 1: 1. In the stapedius the distribution ratio of the red, white and intermediate fibers was approximately 3: 2: 1 or 2: 2: 1.<BR>(3) It is inferred that these three types of muscle fibers which are different trom eacn other in enzymatic activity as well as in the number of mitochondria vary in respiratory activity and metabolic process and accordingly have different physiological functions specific to each type. It is supposed that the red muscle fibers participate in tonic contraction, while the white muscle fibers participate in phasic movement. Muscle fibers of the intermediate type are supposed to have similar function to that of red fibers, judging from their enzymatic activity.(4) Concerning the function of the middle ear muscles, it is p resumed that the red muscle fibers take part in the maintenance and adjustment of specific functional condition of the sound conduction system by giving continuously active tonus to the ossicular chain, while the white muscle fibers participate in rapid contraction in case of middle ear muscle reflexes.