The objectives of the research described in this dissertation were: (1) to develop an assay for determining pancreatic lipase activity in young turkeys by using a glycerol trioleate (GTO) emulsion as substrate and compare it with an assay that utilized a glycerol tributyrate (GTB) emulsion as substrate, (2) to determine pancreatic lipase activity in poults during the first weeks after hatching, (3) to evaluate the effect of 17[beta]-estradiol dipropionate (E[subscript] 2) on pancreatic lipase activity, and (4) to determine the retention of dietary fat as affected by E[subscript] 2 and the type of supplemental fat;Lipase activity (LA, [mu]moles of released fatty acids/min) was linear with both the amount of pancreas added into the medium and the time of incubation up to 20 min. LA was maximal at.5 [mu]moles or more of GTO in the medium. Thus, substrate did not affect the rate of the reaction. Specific lipase activity (SLA, LA per mg of pancreas protein) was consistently greater when determined with the GTO assay than with the GTB assay. Both assays gave results that were very highly correlated (R[superscript] 2 =.80) in 14-day-old poults, but not in 1-day-old poults (R[superscript] 2 =.52). The GTO assay method was recommended for determinations of pancreatic lipase in nutritional studies. SLA averaged 6, 14, and 20 [mu]moles oleic acid (OA)/min x mg of protein at 1, 5, 9, 14, and 21 days of age, respectively. SLA increased rapidly when poults started eating, remained constant until about 14 days, and then increased again by 21 days of age. Poults fed the diet with 10% tallow had greater SLA at 5 and 9 days but lower at 21 days of age than pouts fed the 10% soy-oil diet. Pancreas dry weight was always greater for tallow-fed-poults. Total lipase activity (TLA, [mu]moles OA/min) per pancreas was greater for tallow-fed poults at 5 and 9 days, but no difference occurred by 21 days of age. TLA averaged 100, 700, 1,400, 2,500, and 8,000 [mu]moles OA/min at 1, 5, 9, 14, and 21 days of age, respectively. Injection of 5.5 mg E[subscript] 2/mg of body weight (BW) increased SLA but undesirable side-effects occurred. Injection of.02 E[subscript] 2/mg BW did not have an important effect on SLA. Apparent fat retention averaged 85.3, 84.8, and 93.5% at 5, 9, and 21 days of age, respectively. A positive relationship was found between apparent fat retention and SLA (R[superscript] 2 =.92); however, lipase activity in the pancreas did not seem to be limiting fat retention from the diet in young turkeys.