In previous experiments a considerable interstitial oedema developed after myocardial infarction followed by the infusion of 6 mmol/kg/min norepinephrine lasting for 10 min in the 2nd and 48th hours as well as after a 60 min period of ventricular fibrillation during cardiopulmonary bypass. On the other hand myocardial dehydration was induced by hyperosmolality of 320-470 mOsm/l caused by mannitol or glucose after pancreatectomy. A close bilinear correlation was found between the myocardial water content and the increase of ventricular diastolic stiffness. It was further established that the increase of ventricular stiffness is followed by a decrease of the cardiac output index measured during left ventricular afterload. The cardiac output index was lower after myocardial infarction and pancreatectomy than during cardiopulmonary bypass or mannitol-treatment. Of clinical importance is the fact that ventricular performance decreases when myocardial water content exceeds or does not reach the value of 760-790 g water per 1000 g myocardium.