1. The rate of substrate cycling between acetate and acetyl-CoA in sheep hindlimb muscle was measured from the decrease of [14C]acetate specific radioactivity across the tissue in vivo. 2. The rate of cycling was 0.005-0.015 mumol/min per g and, in two of the three sheep investigated, was approximately doubled as a result of infusing 2 mol acetate/day intravenously. 3. The results indicate that this cycle contributes only approx. 0.5% of the total heat produced by the animal and would probably make only a relatively small contribution to any heat increment of feeding associated with diets producing large amounts of acetate.
