1. The effect of 100 microM (20 micrograms ml-1) of D,L-carnitine was studied on the isolated heart of the rat subjected to 30 min of low flow ischaemia followed by reperfusion. 2. In untreated hearts (n = 30) ischaemia produced an almost total loss of contractility (P less than 0.05 compared with non-ischaemic time control) which was accompanied by an increase in resting tension of approximately 235% (P less than 0.05). Ventricular arrhythmias developed during ischaemia in 100% (P less than 0.05) of untreated hearts studied. Following reperfusion, untreated hearts recovered 16.3% of contractile function and demonstrated a 60% elevation in resting tension. The incidence of reperfusion-associated ventricular fibrillation was 60%. 3. Carnitine treatment produced no effect on either the contractile depression or the elevation in resting tension during ischaemia but did significantly decrease the incidence of arrythmias at the termination of ischaemia to 63.3% (n = 30, P less than 0.05). In the presence of carnitine, contractile recovery at the end of reperfusion was significantly increased to 30.2% (n = 10, P less than 0.05) and the elevation in resting tension was decreased to 30% (n = 10, P greater than 0.05). The incidence of ventricular arrhythmias during reperfusion was significantly reduced by carnitine. 4. Two populations of mitochondria, subsarcolemmal (SLM) and interfibrillar (IFM) isolated at the end of the ischaemic period exhibited an overall increase in oxidative phosphorylation rates as well as uncoupled oxygen consumption; both phenomena were more pronounced with IFM. Carnitine generally potentiated this response. A 29% and 38% inhibition in atractyloside-sensitive ADP uptake was observed in SLM and IFM, respectively, following ischaemia, which was partially prevented by carnitine. 5. After 10min of reperfusion, adenosine diphosphate (ADP) uptake in SLM was further reduced to 55% of control whereas with IFM, uptake was not different from that seen at the end of ischaemia. Mitochondria isolated from hearts after 30 min of reperfusion revealed a significantly depressed oxidative phosphorylation as well as ADP/ATP translocase activity. These defects were partially reversed in hearts perfused with carnitine. 6. Our study demonstrates that D,L-carnitine protects the rat isolated heart against injury associated with ischaemia and reperfusion through a mechanism associated with improved mitochondrial function.