The author studied the ultrastructural changes of the myocardial fibers under the methoxyflurane anesthesia. Nine albino rats were anesthetized with methoxyflurane for 60 minutes. Each of three group were sacrificed immediately, four hours and twelve hours after anesthesia respectively and then heart were extracted out. The results obtained were summarized as follow: First group revealed mild degenerative changes in mitochondria of myocardial fiber. Cristae were decreased in number, and partial disruption, and fused together to form large irregular shaped body, chondriosphere of de Robertis. Sarcoplasmic reticulum was slightly dilated and glycogen particles were also slightly decreased. Second group revealed milder change than first group. The third group revealed to the normal structure.