Twenty-four newly hatched chickens from fast growing line (A) and slow growing line (B) of White Plymouth Rock were measured during the growth period from 1 to 13 days of age. Individual 3-h measurements of O₂ consumption and CO₂ production were carried out in an open-air-circuit respiration unit. The gas exchange data were used to calculate oxidation of carbohydrates (OXCHO) and fat (OXF). Chickens from line A showed better growth performance and higher heat production. The contribution of OXCHO and OXF to heat production showed remarkable differences between lines. From the 4^th day of age, about 90% of the heat was covered by OXCHO in line A, while it decreased steadily from 77 to 29% in line B. The results indicate that in line A, breeding, causing greater feed intake and higher body gains, at the same time has changed the patterns of OXCHO and OXF compared with the slow growing line B.