The objective of this study was to investigate the edema suppresive effect of single-dose crystalloid cardioplegia against immature myocardium. 50 puppies (3-21-day-old) were separated into 4 groups by the method of myocardial preservation, group A: preservation at 30°C, group B: topical cooling used only, group C: topical cooling with cardioplegia (St. Thomas Hospital solution: 4°C, pH 7.8, 350 mOsm/l), group D: topical cooling with oxygenated cardioplegia, and gravimetric water content of myocardium (%) was measured at control, 5, 30, 60, 90, 120, 150, and 180 min after aortic clamp. All hearts had elevated myocardial water content with linear change pattern, although which in groups A and B was consecutively increased while which in groups C and D was increased immediately after aortic clamp followed by slow increase thereafter. Increase of myocardial water content from 5 min after aortic clamp in group B at 90 min was significantly higher (p<0.01) than those in groups C and D, at 180 min that in group A was higher than that in group C and that in group B was higher than those in groups C and D (p<0.05, p<0.01, p<0.01, respectively). This study has shown that evolution of myocardial edema was suppressed by the administration of cardioplegia, while myocardial water content was seemingly higher because coronary vascular dilatation resulted in increase of intravascular water. We could not find the effect of the topical cooling only or oxygenated cardioplegia.