Rat hearts were perfused on a Langendorff apparatus at 37 degrees C with oxygenated Krebs-Henseleit medium with 2.5 mM Ca2+ or Ca2+-free medium for 5 min prior to addition of 0.04 or 1 mM DNP. Effluent was analyzed for creatine kinase (CK) activity and tissue was sampled at 0, 1.5 and 3 min for ATP and CP analysis. Hearts were perfusion-fixed for light and electron microscopy. Contractile function was monitored by intraventricular balloon or a force displacement transducer. In control hearts, 1 mM (but not 0.04 mM) DNP caused a rapid contracture, without CK release. In Ca2+-free hearts, 1 mM (but not 0.04 mM) DNP produced a rapid, massive release of CK. Tissue CP and ATP fell to low levels coincident with contracture and CK release. Morphology of injured hearts showed cellular contracture with uniform sarcomere shortening and widely separated cells with dehiscence of opposing intercalated disc membrane faces. Separated cells had bleb-like protrusions of cytoplasm surrounded by fragmented sarcolemmal membranes. The results demonstrate that following Ca2+-free perfusion, irreversible damage can occur in the absence of extracellular Ca2+ and in hearts with uncoupled mitochondria which were depleted of ATP. It is postulated that Ca2+-free perfusion weakens intercalated discs and that DNP-stimulated mitochondrial calcium release produces contracture, which tears cells apart at weakened cell junctions, causing sarcolemmal membrane damage and CK release.