1. The aim of this study was to determine the effect of glucose supplementation on leucine turnover during and after exercise and whether variation in the previous dietary protein content modulated this effect. 2. Postabsorptive subjects received a primed constant [1-C-13,N-15]leucine infusion for 6 h, after previous consumption of a high (1.8 g kg(-1) day(-1) HP, n = 16) or low (0.7 g kg(-1) day(-1), LP, n = 16) protein diet for 7 days. The subjects were studied at rest; during 2 h of exercise, during which half of the subjects from each dietary protocol received 0.75 g kg(-1) h(-1) glucose (HP + G, LP + G) and the other half received water (HP + W, LP + W); then again for 2 h of rest. 3. Glucose supplementation suppressed leucine oxidation (P <0.01) by 20% in subjects consuming the high protein diet (58.2 +/- 2.8 mu mol kg(-1) h(-1), HP + G; 72.4 +/- 3.9 mu mol kg(-1) h(-1) HP + W) but not the low protein diet (51.1 +/- 5.9 mu mol kg(-1) h(-1), LP + G; 51.7 +/- 5.5 mu mol kg(-1) h(-1), LP + W), with no difference in skeletal muscle branched-chain 2-oxo acid dehydrogenase (BCOADH) activity between groups. Glucose supplementation did not alter the rate of whole-body protein synthesis or breakdown. 4. The sparing effect of glucose on leucine oxidation appears only to occur if previous protein intake was high. It was not mediated by a suppression of BCOADH fractional activity but may be due to reduced substrate availability.