The study was conducted on 476 male Shaver Starbro chickens, divided into four groups and fed ad libitum diets differing in their lard content: 0, 3, 6, or 9%. On days 1, 2, 3,4, 7, 12, and 21 days of life the chickens were weighed, some were killed, the yolk sac was weighed, and its crude protein and fat contents were determined. Amylase and lipase activity was assayed in the blood and pancreas. The chickens fed fat-supplemented rations were heavier on day 21 of life. No significant differences were found, however, between groups in the decrease in yolk sac weight. A high fat content of the diet (9%) slowed down the rate at which fat was resorbed from the yolk sac, but the chickens developed normally. Resorption of protein from the yolk sac by chickens given the unsupplemented feed was uniform. At a higher lard content in the diet, a tendency appeared towards reducing the rate of lipid resorption from the yolk sac; this process intensified after day 4 of life. The addition of lard to the diet significantly lowered the activity of lipase in the pancreas on 3 day of life and its gradual increase was observed between days 7 and 21. The addition of fat to the diets at levels from 6 to 9% decreased the activity of pancreatic amylase on days 3 and 7 of life.