1. 1. Metmyoglobin reacted with H2O2 to form ferryl-myoglobin, which reverted back spontaneously to the met-form. 2. 2. Through this cyclic reaction of myoglobin between metMb(III) and ferryl-Mb(IV), we propose that H2O2, one of the potent oxidants in vivo, can be decomposed continuously in cardiac and skeletal muscle tissues in the absence of catalase and peroxidase.