Abstract: Replacement of dietary saturated fatty acids (SFA) by polyunsaturated fatty acids (PUFA) has been consistently shown to reduce the amount of abdominal fat in broiler chickens, but the metabolic basis for this effect is unknown. It was hypothesized that the feeding of PUFA instead of SFA would inhibit whole-body de novo fatty acid synthesis. As indexes of de novo fatty acid synthesis, we used the concentration of plasma triacylglycerols and minimum fatty acid synthesis calculated as fatty deposition minus digestible fatty acid intake. Broiler chickens were given one of five diets in which the beef tallow component, which is rich in SFA, was replaced by increasing amounts of soybean oil, which is rich in PUFA. The variable fat content of the diets was 3% (w/w). There were neither significant nor systematic effects on weight gain and feed:gain ratio. The amount of abdominal fat was reduced significantly when about 75% of the tallow was replaced by soybean oil, but there was no further decrease after the incorporation of more soybean oil into the diet. The decrease in abdominal fat was associated with a decrease in the level of plasma triacylglycerols, but it was not associated with minimum de novo fatty acid synthesis in the whole body.