Cytochrome oxidase activity was investigated histochemically in the choroid plexus epithelium. Intense staining for the enzyme was exclusively limited to the mitochondria. Rats treated with octanoic acid displayed extensive ultrastructural disruptions in the epithelial cells of the choroid plexus. Mitochondria were fewer in number and more disrupted compared to the control. The enzyme activity was greatly reduced. However, pretreatment with an equimolar dose of L-carnitine followed by octanoic acid injection produced little alteration of either ultrastructure or enzyme staining. This study suggests that L-carnitine supplementation may restore mitochondrial function of the choroid plexus subjected to toxic organic anions in metabolic disorders, and may be useful in the prevention of metabolic encephalopathy.