The effects of chronic amiodarone therapy on myocardial phospholipid hydrolysis induced by total in vitro ischaemia were investigated in cat hearts. Chronic treatment of cats with amiodarone (30 mg/kg/day, orally) for 6 weeks resulted in a sufficient uptake of the drug reaching tissue levels of 83 +/- 13 & 122 +/- 22 microM (n = 12) for amiodarone and its principle metabolite, desethylamiodarone, respectively. This was accompanied by a significant increase (37%, P less than 0.001) in total phospholipid content of heart in treated as compared to untreated animals. Upon in vitro total ischaemia, these endogenous drug levels were sufficient to attenuate significantly hydrolysis of membrane phospholipid. The degree of attenuation was dependent upon the duration of ischaemic insult. In this regard, protection against phospholipid losses by amiodarone treatment was significantly more in the later irreversible phase of ischaemic injury whether studied in an in vitro total ischaemia model or in an isolated perfused heart preparation. Similar trend was observed in the relative accumulation of lysophospholipid and non-esterified fatty acid levels during ischaemia, i.e. both were significantly attenuated by amiodarone treatment. However, in contrast to the fatty acid data, the net changes in lysophospholipids per gram tissue wet weight were similar in treated and untreated animals, suggesting that the protective effects of amiodarone may have involved other enzymes including phospholipase C and D. Also, during the entire time course studied, all the phospholipid classes appeared to be affected to more or less a similar degree, indicating that the effects of the drug may have manifested in other subcellular compartments besides lysosomes. However, at all time periods studied, the net release of eicosatetraenoic and docosahexaenoic acid (fatty acids occupying primarily sn-2 position of phospholipids) was different, release of the former fatty acid being inhibited more than the latter, suggesting specific interaction of amiodarone with the molecular species of phospholipid. The data suggest that amiodarone attenuates ischaemia-induced membrane lipid abnormalities in part through modulation of phospholipid metabolism, and that this effect may be one of the key determinants which contribute to its antiarrhythmic properties during acute ischaemia.