The purpose of the present study was to measure the activity of carnitine palmitoyltransferase (CPT) in the lateral and fourth ventricular choroid plexus (LVCP, FVCP) as an example of an enzyme committed to mitochondrial long-chain fatty acid oxidation. The CPT activity measured with both assays determines total CPT activity, that is, the activity of both CPT-A (outer form) and CPT-B (inner form). CPT-assay 1 (forward reaction) activity was 893.3 +/- 44.5 mU/g and 803.3 +/- 71.3 mU/g (mean +/- S.E.M.) in LVCP and FVCP, respectively. CPT-assay 2 (backward reaction) activity was 3673.7 +/- 92.4 and 3417.0 +/- 277.1 mU/g in LVCP and FVCP, respectively. These data demonstrate that CPT activity is present in the rabbit choroid plexus, and that the activity is somewhat higher per gram wet weight of tissue than the activity observed in the skeletal muscle (CPT-assay 1 = 514.4 +/- 159, CPT-assay 2 = 2492 +/- 576 mU/g). CPT activity has been exclusively localized to the mitochondria of the liver, heart, kidney and the skeletal muscles. Our present study further demonstrates that the choroid plexus is another organ in which high activity of CPT exists.