The experiment was carried out on 476 Starbro chickens (♂) divided into 4 groups, each in seven replications with 17 birds kept in cages in standard enviroment conditions. Chickens were fed mixtures contained 21.7% (starter), 20.0% (grower) crude protein and 9, 6, 3 or 0% of rape seed oil. In 1, 2, 3, 4, 7, 12, 21 and 35 days of life and 2 h after feeding chickens were weighted and killed. The resorption rate of yolk sac and changes of α-amylase and lipase activity in blood serum and pancreas were estimated. The body weight of chickens was the best in group II, III and IV. Highest level of oil in mixture has reduced the growth after 21 days of life. Addition of fat to feed mixtures for chickens in their first 21 days of life influences the rate of yolk sac resorption, decreases feed intake and activity of α-amylase in the pancreas, while conversely lipase activity in the pancreas increases together with increase in quantity of rape seed oil added.