The ATP/IMP ratio of stressed fish muscle in summer (from June to August) was lower at the initial stage than the ratio in winter (from January to March), and reached to the ultimate value 3h earlier. Though stressed fish muscle in summer showed larger contraction than that in winter, almost the same tension values were observed in both seasons. This result was ascribed to the structure weakening due to large contraction in summer.
The ATP /IMP ratio of unstressed fish muscle in summer was lower at the initial stage than the ratio in winter, and reached to the ultimate value 4h earlier. The initial ATP /IMP ratios of spinal cord destroyed fish muscles at two seasons were both high values. The ratio in summer was slightly lower at the initial stage than the ratio in winter, and reached to the ultimate value 10h earlier. Unstressed and spinal cord destroyed fish muscles in summer contracted more than those in winter, and showed larger tension values. This result means that the structure weakening was not remarkably induced in the muscles obtained in summer. In spinal cord destroyed fish muscles the slow but continuous tension development induced the structure weakening. Therefore the tension increment in summer was not as large as in unstressed fish muscle.