Two experiments were conducted to study the interaction between breeder age and egg size on the energy utilization (experiment 1) and heat production (experiment 2) of broiler embryos. In experiment 1, a total of 4,800 Ross-308 hatching eggs from 2 breeder ages (29 and 53 wk of age, or young and old) and, within each age, 2 egg sizes (57 to 61 g and 66 to 70 g, or small and large) were used. In experiment 2, a total of 240 Ross-308 hatching eggs from 2 breeder flocks at 29 (young) and 53 (old) wk of age, and which were selected from the same egg weight range (58 to 61 g), were tested in 2 replicate chambers. In experiment 1, it was shown that the amount of yolk relative to albumen was higher in the old flock eggs, and this effect was more pronounced in the large eggs. The old flock eggs, especially the larger egg size, contained more energy as a result of a greater yolk size. Energy utilization of the embryos was positively related to yolk size and the amount of energy transferred to yolk-free body (YFB) was largely determined by the available egg energy. The efficiency of converting egg energy into chick body energy (E(YFB)) was equal for both egg sizes and both breeder age groups. Chick YFB weight of young and old flock eggs was equal. However, dry YFB weight of chicks from old flock eggs was higher than in chicks from young flock eggs, which was associated with more protein and fat content and thus more energy accumulated into YFB. As a consequence, embryos derived from old flock eggs produced more heat from d 16 of incubation onward than those of the young flock eggs. In conclusion, the higher energy deposition into chick YFB of old flock eggs, leading to higher embryonic heat production, is the result of a higher amount of available energy in the egg and is not due to changes in E(YFB).