1. Idioventricular rate responses and adrenoceptor number have been examined in normal Langendorff hearts perfused at 70 cmH2O and in 'ischaemic' hearts perfused at 10 cmH2O. 2. In hearts perfused at normal pressure, ventricular rate responses to phenylephrine in the presence of propranolol were biphasic with low concentrations of phenylephrine reducing, and high concentrations increasing, ventricular rate. Both responses were abolished in the presence of prazosin (100 nM). In hearts perfused at low pressure, the negative chronotropic responses to phenylephrine were no longer present and positive chronotropic responses were enhanced compared with those of control tissues. The number of ventricular [3H]-prazosin binding sites was also significantly increased during ischaemia. 3. Idioventricular rate responses to isoprenaline were depressed in ischaemic tissues compared with controls, but [3H]-dihydroalprenolol binding was not altered in these hearts. 4. The incidence and duration of arrhythmias which occurred during 30 min of global ischaemia and reperfusion were not significantly altered in the presence of 100 nM prazosin. 5. These results demonstrate that reducing the perfusion pressure of rat isolated hearts enhances alpha-adrenoceptor-mediated responses and increases alpha-adrenoceptor density. Whether the increase in alpha-adrenoceptor responsiveness during ischaemia is involved in arrhythmogenesis requires further investigation.