Adequate energy status in lymphocytes is vital for their development. The ability of developing chicken lymphocytes to acquire and metabolize energy substrates was determined during embryonic days (e) and neonatal days (d) of life when primary-energy substrate metabolism is altered at the whole-animal level. In 3 experiments, bursacytes and thymocytes were isolated on e17, e20, d1, d3, d7, or d14 to analyze markers associated with glucose, glutamine, and lipid metabolism. Bursacyte glucose transporter-3 (Glut-3) mRNA abundance increased from d1 to d14 and hexokinase-1 (HK-1) mRNA abundance was maximum on e20 (P < 0.05). Thymocyte Glut-1, Glut-3, and HK-1 mRNA abundance increased from e17 to d14 (P < 0.05). HK enzyme activity increased from e20 to d3 in bursacytes and d3 to d7 in thymocytes (P < 0.05). Glucose uptake by bursacytes and thymocytes was greater on d14 compared to d1 and d7 (P < 0.05). Bursacyte and thymocyte sodium coupled neutral amino acid transporter-2 and glutaminase (GA) mRNA abundance increased from e20 to d7 (P < 0.05). GA enzyme activity increased from e20 to d7 in bursacytes (P < 0.05) and did not change in thymocytes. Carnitine palmitoyl transferase enzyme activity did not change over time in either cell type. These studies suggest that developing B and T lymphocytes adapt their metabolism during the first 2 wk after hatch. Developing lymphocytes increase glucose metabolism with no change in fatty acid metabolism and bursacytes, but not thymocytes, increase glutamine metabolism. Understanding the factors that regulate lymphocyte development in neonatal chicks may help promote their adaptive immune responses to pathogens in early life. J. Nutr. 137: 427-432, 2007.