We describe the measurement of carnitine and short-chain and long-chain acylcarnitines in perchloric acid extracts of liver and muscle. [1-14C]Acetyl-CoA is used, and labeled acetylcarnitine is measured with a sensitivity of less than 1 to 50 nmol of L-carnitine per 50-microliter sample. Samples frozen at -20 degrees C lost no carnitine during several months. Analytical recovery of carnitine (5 nmol) added to 50-microliter perchloric acid extracts of liver was 103 +/- 2%. This method may be suitable for clinical applications such as screening patients with muscle and metabolic disorders.