Cardiac transplantation is a prominent development in the treatment of patients with severe cardiac diseases. If it is not possible to transplant the heart immediately after removing it from the donor, procedures for preparing, protecting, storing and transferring of the heart constitute a major portion of this study. Selecting the best method among those used for cardioprotection is still been debated. This experimental study investigated whether isocolloidoosmotic solution in addition to St. Thomas Hospital cardioplegic solution would give better cardioprotection. Wistar rat hearts were isolated and perfused by the Langendorff method (n = 6 for each group). In the control group after stabilisation, the hearts were arrested while perfused with St. Thomas Hospital cardioplegic solution for 3 min, then they were placed in cardioplegic solution at 4 degrees C for 6 h. Afterwards the hearts were reperfused. In the experimental groups, modified gelatin fluid (30 g/l) or HES 200/0.5 (50 g/l) or dextran 70 (25 g/l) was added to the cardioplegic solution. Maximum contractility, +dF/dt(max), -dF/dt(max), heart rate, contractility rate product, coronary flow and lactate dehydrogenase, creatine phosphokinase enzyme leakage were measured in all groups during pre-ischemic and post-ischemic periods (10 min after reperfusion). At the end of each experiment, the hearts were weighted and tissue levels of lipid peroxide, expressed in terms of thiobarbituric acid reactive substances, malondialdehyde, glutathione (an important intracellular antioxidant) and ATP were measured. Non-ischemic tissue levels of malondialdehyde, glutathione and ATP were also measured in another group (n = 6). There was no statistically significant difference among the simultaneous experimental and control groups in any criteria evaluated (P > 0.05). The addition of synthetic colloids to the standard cardioplegic solution did not provide further protection except for the gelatin group in which recovered contractile force was not significantly different from the group's initial values. This effect may be explained by its degradation to amino acids which may play a substrate role. (C) 1997 The Italian Pharmacological Society.