Abstract: Effects of acute (meal) and chronic (diet) level of protein supply on metabolic leucine utilization were investigated in growing (10 weeks) and mature (> 1 year) rats. Rats were conditioned on a high-protein (HP) diet (210 g casein/kg feed) or a low-protein (LP) diet (75 g casein/kg feed) from 7 weeks of age. Overnight-fasted rats were offered a HP or LP meal during a 8 h 14CO2 breath test with a constant infusion of either L-[1-14C]leucine (carboxyl, CL) or L-[U-14C]leucine (universal, UL). Before the meal 14CO2 output was lower for overnight-fasted rats fed on LP than on HP (P <0.001), and also lower for growing than for mature rats (P <0.001). Meal ingestion resulted in a rapid increase in 14CO2 output. From 2 h after the start of the meal the effect of acute protein supply on 14CO2 output was significant (P <0.001), while the effect of chronic protein supply disappeared for CL. After the meal 14CO2 output was transiently lower for growing than for mature rats (P <0.05), especially after the LP meal. The difference in 14CO2 output between CL and UL increased transiently after the meal, indicating an increase in decarboxylation relative to total oxidation of leucine.