The emperor penguin (Aptenodytes forsteri) is an Antarctic seabird feeding mainly on fish and therefore has a high dietary intake of n-3 polyunsaturated fatty acids. The yolk is accumulated in the developing oocyte while the females are fasting, and a large proportion of the fatty acid components of the yolk lipids are derived by mobilization from the female's adipose tissue. The fatty acid composition of the total lipid of the yolk was characterized by high levels of n-3 polyunsaturated fatty acids. However, it differed in several respects from that of the maternal adipose tissue. For example, the proportions of 14:0, 16:1n-7, 20:1n-9, 22:1n-9, 20:5n-3, and 22:6n-3 were significantly greater in adipose tissue than in yolk. Thus adipose tissue lipids contained 7.6+/-0.3% and 8.0+/-0.3% (wt% of total fatty acids; mean +/- SE; n = 5) of 20:5n-3 and 22:6n-3, respectively, whereas the yolk total lipid contained 1.6+/-0.1 and 5.5+/-0.3% of these respective fatty acids. The proportions of 16:0, 18:0, 18:1n-9, 18:2n-6, and 20:4n-6 were significantly lower in the adipose tissue than in the yolk lipids. The proportions of triacylglycerol, phospholipid, free cholesterol, and cholesteryl ester in the yolk lipid were, respectively, 67.0+/-0.2, 25.4+/-0.3, 5.3+/-0.2, and 1.8+/-0.2% (wt% of total yolk lipid). The proportions of 20:4n-6, 20:5n-3, 22:5n-3, and 22:6n-3 were, respectively, 5.7+/-0.3, 2.8+/-0.2, 1.4+/-0.1, and 11.7+/-0.5% in phospholipid and 0.4+/-0.0, 1.2+/-0.1, 0.8+/-0.1 and 3.6+/-0.3% in triacylglycerol. About 95% of the total vitamin E in the yolks was in the form of alpha-tocopherol with gamma-tocopherol forming the remainder. Two species of carotenoids, one identified as lutein, were present.