The cardioprotective effects of mioflazine, a recently developed cardiovascular drug, were investigated in 41 anaesthetised open chest Beagle dogs subjected to 1 h normothermic global myocardial ischaemia. The severity of the model is evidenced by the finding that only one out of 20 control dogs could be weaned from extracorporeal bypass. Oral pretreatment with mioflazine (2.5 mg X kg-1) resulted in complete functional recovery in 17 out of 20 animals. Biochemical analysis of left ventricular biopsies taken before, during and after aortic cross clamping showed a preservation of purines and a better recovery of ATP, ATP/ADP X Pi ratio and energy charge (p less than 0.05) in the pretreated animals. Morphological and cytochemical assessment of the myocardium demonstrated that the ultrastructure of the sarcolemma and its calcium binding capacity is remarkably well preserved in the drug treated animals. These results indicate a strong cardioprotective effect of mioflazine. The biochemical, cytochemical and ultrastructural findings suggest an interaction of the drug with the sarcolemma.