The study aims to determine a possible relationship between intracellular water, energy metabolism, functional recovery and membrane permeability, during and after hypothermic cardiac preservation. Isolated rat hearts were stored for 12 h at 4°C with University of Wisconsin (UW), St Thomas Hospital (ST) and Krebs–Henseleit (KH) solutions, and were reperfused for 1 h. Cellular volumes were measured by¹H NMR of water and⁵⁹Co NMR of the extracellular marker cobalticyanide, and energetic profiles by³¹P NMR spectroscopy. Storage in ST solution reduced ischemic swelling from 2.50±0.06 to 2.73±0.09 (P<0.001v3.56±0.10 ml/g dry weight in KH), while UW solution caused cellular shrinkage to 2.12±0.08 ml/g dry weight. Intracellular ATP concentrations and pH values were higher in UW as compared to ST solution. At reperfusion, hearts stored in ST shrank while those stored in UW expanded, resulting in similar intracellular volumes. Storage with UW was superior to ST in post-ischemic function 65±5% (P<0.01v49±4% with ST) and in recovery of ATP 46±3% (P<0.001v25±4% with ST). Storage with both ST and UW solutions did not prevent interstitial edema. Sarcolemmal membrane integrity, as assessed by cellular swelling in response to a hypo-osmotic shock (210 mmol/l), was significantly improved by ST and UW solutions as compared to KH (P<0.05). Creatine kinase efflux was reduced by ST and UW as compared to KH (P<0.05), and by UW as compared to ST (P<0.05). Coronary flow was higher following storage with UW than ST solutions, 66±6 and 45±4%, respectively (P<0.01). According to these data, the beneficial effects of UW and ST solutions on hypothermic ischemic storage of rat hearts included prevention of cellular edema and preservation of sarcolemmal membrane integrity. It is concluded: (a) UW and ST solutions reduce ischemic and reperfusion cellular volumes; (b) both solutions, and UW in particular were efficient in preservation of membrane integrity; (c) prevention of cellular edema is not the single or main mechanism responsible for the improved preservation with UW and ST solutions.