1. The importance of circulating non-esterified fatty acids as a substrate during and after low-grade exercise has been examined by using a nicotinic acid analogue to inhibit lipolysis. Seven healthy men received acipimox or placebo on separate occasions. After 90 min, bicycle exercise was performed for 45 min (40% of pre-determined maximum oxygen uptake), followed by a 60 min recovery period. 2. The plasma concentration of non-esterified fatty acids increased during exercise after placebo (320 ± 80 to 630 ± 110 μmol/l) and remained elevated in the post-exercise period. Basal concentrations were lower after acipimox (100 ± 10 μmol/l; P < 0.05); they declined to 60 ± 10 μmol/l during exercise and remained at this level for the rest of the study. 3. Lipid oxidation increased from 0.8 ± 0.1 to 4.2 ± 0.5 mg min −1 kg −1 during exercise after placebo ( P < 0.001) and remained elevated in the post-exercise period (1.2 ± 0.1 mg min −1 kg −1 ). It was lower after acipimox, but still increased from 0.3 ± 0.1 to 2.3 ± 0.2 mg min −1 kg −1 with exercise. Carbohydrate oxidation was increased after acipimox compared with after placebo, but only reached significance during the post-exercise period ( P < 0.05). 4. Although acipimox abolished the rise in the plasma concentration of non-esterified fatty acids during exercise, there was only a 50% decrease in the rate of lipid oxidation. This suggests that an alternative source of non-esterified fatty acids makes an important contribution to the supply of lipid for oxidation during exercise. The elevated plasma concentration of non-esterified fatty acids and lipid oxidation after exercise serve to limit the further oxidation of carbohydrate.