Two groups of rats were fed diets containing large amounts (45-50% of the total digestible energy) of sunflower seed oil or hydrogenated coconut oil for 4-5 days. The left ventricular working capacity, the coronary flow rate, the oxygen consumption, the glucose uptake and the lactate release were determined in the isolated perfused heart. The fatty acid composition of the heart phospholipids was also determined. The left ventricular working capacity and the coronary flow rate of hearts of rats fed sunflower seed oil are higher (10-20%) than those of rats fed hydrogenated coconut oil. Feeding the two fats for 3-4 weeks instead of 4-5 days does not alter the results. There are no or only minor differences between the two dietary groups as to the other quantities mentioned. It is concluded that dietary fats affect the properties of the heart already after a short feeding time.