Abstract: A study to determine the effects of dietary fat composition and post-hatching age on lipid changes of the yolk sac membrane (YSM) and the liver. Two groups of ninety-day-old male broiler chicks were fed diets containing a saturated fat (tallow oil) Diet 1 and unsaturated fat (soybean oil) Diet 2 for two weeks. Twelve birds from each treatment were sacrificed on days 1, 3, 6, 9, 12 and 14 post-hatch and samples of the YSM, liver and gall bladder bile were collected. Weight changes of the chick and tissues and also lipid composition were determined during the experimental period. About 75% decrease in YSM weight occurred during the first 3 days post-hatch and it was negligible by day 9 post-hatch in both groups. Triglycerides (TG) was the major lipid component of the YSM at day 1 post-hatch (>60% of total lipid) but had declined to less than 2% on day 12 post-hatch. The decreases in TG were accompanied by significant increases of cholesterol esters (CE). These changes were not affected by dietary treatment. The liver constituted 3 and 6% of chick body weight at hatching and at day 6 post-hatch respectively and remained constant thereafter. Changes in total lipid content and lipid composition were noted and were influenced by both dietary fat composition and post-hatching age. It was concluded from these this study major lipid compositional changes occur in the chick body tissues following hatching and these changes are influenced to some extent by dietary composition.