Adductor muscles dissected from live scallops (Patinopecten yessoensis) were preserved in artificial seawater containing various concentrations of dissolved oxygen at 5 degreesC. The period over that the initial ATP level was maintained decreased with a decrease in oxygen concentration. The K-value, an index of freshness, rose markedly after ATP levels and energy charge decreased. Muscle contraction occurred after 4 to 5 d in 1.0 mM oxygen or after 3 d in 0.07 mM oxygen. Sodium azide and 2,4-dinitrophenol accelerated the decrease in ATP, even in oxygenated seawater. These results suggest that enough oxygen to support aerobic respiration is required to preserve adductor muscles in a vital state for a relatively long period.