The lipid composition of the sarcolemma influences its function. The purpose of this study is to investigate the electrophysiological alterations induced in the rat's heart by dietary manipulation of cardiac fatty-acid composition. Strength-duration and strength-interval relationships were used to study excitability and refractoriness respectively. Ventricular fibrillation threshold measured by short bursts of rapid stimulation was used to indicate ventricular vulnerability. Gas liquid chromatography was used to analyse cardiac fatty-acid-composition. We used two-week-old rats fed with fat-free diet and 5% and 10% soya bean oil-supplemented diets for 6-9 weeks. Fat-free diet only was associated with increased eicosatrienoic acid content, a marker of essential fatty-acid deficiency. A decrease in poly-unsaturated to saturated (P:S) fatty-acid ratio was also observed. Five percent oil supplementation was associated with a delay and attenuation of this effect. Fat-free diet was associated with increased excitability, decreased refractoriness and reduced ventricular fibrillation threshold. Ten percent oil supplementation prevented these effects while 5% oil supplementation had only a temporary protective effect. We conclude that manipulation of dietary fat content can affect cardiac fatty-acid composition and electrophysiology.