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The objective of this dissertation research was to investigate the effect of dietary macronutrient composition on neuropeptide Y (NPY)-mediated regulation of adipose tissue physiology in broiler chickens during the early post-hatch period. A high-carbohydrate (HC), high-fat (HF) or high-protein (HP) diet was fed to broiler chicks in all experiments and various facets of physiology were evaluated at day 4 post-hatch, including diet-, fasting-, and neuropeptide Y-induced effects on gene expression, cellular morphology, and lipid metabolism. Experiment 1 was designed to study the effects of diet on molecular changes in different adipose tissue depots (subcutaneous, clavicular and abdominal) after 3 hours of fasting and 1 hour of refeeding. Adipose tissue weights were decreased in chicks that consumed the HP diet, whereas adipocyte diameter was increased in response to the HF diet. There was greater expression of mRNAs encoding fatty acid binding protein 4 (FABP4) and monoglyceride lipase in chicks fed the HC and HF diets than the HP diet in all three adipose tissue depots. Fasting increased plasma non-esterified fatty acid concentrations in chicks fed the HC and HP diets. Results suggest that the heavier fat depots and larger adipocytes in chicks fed the HF diet are explained by greater rates of hypertrophy, whereas the HP diet led to a decrease in adipose tissue deposition, likely as a result of decreased rates of adipogenesis. Experiments 2 and 3 were designed to investigate how dietary macronutrient composition affects the effect of centrally or peripherally administered NPY, respectively, on lipid metabolism-associated factor mRNAs in adipose tissue. In experiment 2, vehicle or 0.2 nmol of NPY was injected intracerebroventricularly (ICV) and abdominal and subcutaneous fat samples were collected at 1 hour post-injection. In the subcutaneous fat, ICV NPY injection decreased peroxisome proliferator-activated receptor gamma (PPAR gamma) and sterol regulatory element-binding transcription factor 1 (SREBP1) mRNAs in chicks fed the HF diet, whereas there was an increase in SREBP1 expression in chicks fed the HF diet after NPY injection. Expression of PPAR gamma and FABP4 mRNAs increased in the abdominal fat of HF diet-fed chicks after NPY injection. Thus, HF diet consumption may have enhanced the sensitivity of chick adipose tissue to the effect of centrally-injected NPY on gene expression of adipogenesis-associated factors. In experiment 3, vehicle, 60, or 120 micrograms/kg BW of NPY was injected intraperitoneally (IP), and subcutaneous, clavicular, and abdominal fat was collected at 1 and 3 hours post-injection. Food intake and plasma NEFA concentrations were not different among chicks fed the HC, HF or HP diet after IP NPY injection, indicating that the effects of NPY on adipogenesis were independent of secondary effects due to altered energy intake. In response to the lower dose of NPY, the expression of NPY receptor sub-type 2 mRNA was increased at 1 hour post-injection in the subcutaneous fat of chicks fed the HP diet, whereas there was less 1-acylglycerol-3-phosphate O-acyltransferase 2 mRNA in the subcutaneous fat of chicks fed the HC diet. The higher dose of NPY was associated with greater AGPAT2 mRNA in the clavicular fat of chicks that consumed the HP diet and less CCAAT/enhancer-binding protein alpha in the abdominal fat of chicks that were provided the HF diet. However, there was also a decrease in the expression of some of these factors, although mechanisms are unclear. In conclusion, dietary macronutrient composition influenced the response of adipose tissue to the adipogenic effects of NPY and metabolic effects of short-term fasting and refeeding during the first week post-hatch. Collectively, this research may provide insights on understanding NPY's effects on the development of adipose tissue during the early life period and mechanisms underlying diet-dependent and depot-dependent differences in adipose tissue physiology across species. Ph. D.