Myocardial lipoprotein lipase (LPL) activity during ischaemia has not been fully understood, although it plays an important role in regulating myocardial fatty acid metabolism. In this experiment, the effects of ischaemia (Experiment A) and anoxia (Experiment B) on two distinct fractions of LPL, i.e. functional and nonfunctional forms were investigated in isolated, perfused rat heart. In Experiment A, hearts were perfused by Neely-Morgan working heart mode with Krebs Henseleit bicarbonate (KHB) buffer (95% O2:5% CO2), then whole heart ischaemia was induced by using a one-way aortic valve, simultaneously switching to the same buffer but containing heparin (5 mu/ml) for 20 min. In Experiment B, the hearts were perfused by Langendorff method with KHB buffer (95% O2:5% CO2) and the buffer was switched to KHB (95% N2:5% CO2) containing heparin for 20 min. Coronary effluent was collected at 5 min intervals and used for the measurement of functional LPL activities using 3H-glyceryltrioleate. The hearts were quickly frozen at the end of perfusion and homogenized. The suspension was used for the measurement of non-functional LPL activities. In Experiment A, functional LPL activities in ischaemia were significantly lower than in the aerobic condition. On the contrary, the nonfunctional LPL activity in ischaemia was significantly higher than in the aerobic condition. In Experiment B these values were also significantly lower than in the aerobic condition. However, there was no significant difference in non-functional LPL activity between the anoxia and aerobic condition. These results indicate that there is a conversion defect from the precursor of LPL to the functional form of LPL in ischaemia, whereas disturbed LPL synthesis might be involved in anoxic myocardium.