To determine morphological changes in the very early stage of damage and recovery of myocardial fibers in relation to electrolytes and water content in the myocardium, experimental myocardial infarcts were produced after injection of 75 mg/kg isoproterenol (Isp) to rats weighing 270 +/- 10 g. Electrolytes were examined by an atomic absorption spectrophotometer, and histologic examination was performed by light and electron microscopy. At 30 min after Isp injection, Ca and water contents of myocardium increased, but Mg decreased. These myocardial damages showed a maximum level from 6 to 12 h after injection of Isp. It consisted of reversible injury of muscle fibers represented by eosinophilic and fatty degeneration, intermingled with coagulation necrosis of fibers which was irreversible. Most of these eosinophilic degeneration fibers recovered to normal fibers between 12 h and 3 d after the injection. The chronological changes of Ca, Mg, and water contents were well correlated to the morphological early changes of myocardial fibers, and this experimental model by using Isp may be useful for examining the recovery of damaged myocardial tissue and estimating the effects of therapeutics.