Abstract 1. 1. The effect of a 48 h fast on the acylation rates of carnitine and glycerophosphate by isolated liver mitochondria and microsomes is measured, in the presence of ATP and CoASH, with palmitate (complexed to albumin) as the acyl donor. 2. 2. During fasting the acylation rate of carnitine increases 2–3 fold in the microsomal fraction and by a small percentage in the mitochondria. It is not inhibited by palmitoyl-CoA at concentrations up to 0.1 mM. 3. 3. During fasting the acylation rate of glycerophosphate decreases by 33% in isolated mitochondria and by 13% in microsomes. High concentrations of palmitoyl-CoA (above 0.02 mM) inhibit the reaction. 4. 4. The significance of these results for the partition of long-chain fatty acids between the esterification and oxidation pathways is discussed.