Recent studies demonstrate a strong link between the type of fat consumed and arrhythmogenesis and implicate altered production of eicosanoids as a potential mechanism. Thus, in a preliminary study designed to evaluate this hypothesis, five dietary lipid supplements - sunflower seed oil (SSO), chemically refined palm oil (PO I), physically refined palm oil (PO II), sheep fat (SF), and a 1:1 blend of SF and fish oil (SF-FO) - were compared for their effects on membrane-lipid composition and eicosanoid synthesis after a 12-mo prefeeding period. Polyunsaturated fatty acids (PUFAs) of the myocardium showed significant alterations in response to dietary lipid treatment. Nevertheless, prostacyclin production was unaffected whereas both SF-FO and POI diets caused a significant inhibition of myocardial thromboxane A2 (TXA2). It is speculated that n-3 (ω-3) PUFAs may act as specific inhibitors of TXA2 synthetase whereas the PO I effect is unlikely to be mediated via fatty acids. These preliminary data are worthy of further investigation.