The effects of carbohydrate deprivation on the regulation of pyruvate dehydrogenase (PDH) were studied at rest and during moderate-intensity exercise. An inhibitory effect of a chronic low-carbohydrate diet (LCD) on the active form of PDH (PDHa) mediated by a stable increase in PDH kinase (PDHK) activity has recently been reported (Peters SJ, Howlett RA, St. Amand TA, Heigenhauser GJF, and Spriet LL. Am J Physiol Endocrinol Metab 275: E980-E986, 1998.). In the present study, seven males cycled at 65% maximal O2 uptake for 30 min after a 6-day LCD. Exercise was repeated 1 wk later after a mixed diet (MD). Muscle biopsies were sampled from the vastus lateralis at rest and at 2 and 30 min of exercise. At rest, PDHa activity (0.18 ± 0.04 vs. 0.63 ± 0.18 mmol · min−1 · kg wet wt−1), muscle glycogen content (310.2 ± 36.9 vs. 563.9 ± 32.6 mmol/kg dry wt), and muscle lactate content (2.6 ± 0.3 vs. 4.2 ± 0.6 mmol/kg dry wt) were significantly lower after the LCD. Resting muscle acetyl-CoA (10.8 ± 1.9 vs. 7.4 ± 0.8 μmol/kg dry wt) and acetylcarnitine (5.3 ± 1.4 vs. 1.6 ± 0.3 mmol/kg dry wt) contents were significantly elevated after the LCD. During exercise, PDHa, glycogenolytic rate (LCD 5.8 ± 0.4 vs. MD 6.9 ± 0.2 mmol · min−1 · kg dry wt−1 ), and muscle concentrations of acetylcarnitine, pyruvate, and lactate increased to the same extent in both conditions. The results of the present study suggest that inhibition of resting PDH by elevated PDHK activity after a LCD may be overridden by the availability of muscle pyruvate during exercise.