Okadaic acid parallely increased carnitine [corrected] palmitoyltransferase I activity and the rate of palmitate oxidation in isolated rat hepatocytes. Nevertheless, okadaic acid had no significant effect on the rate of octanoate oxidation. Maximal effects of okadaic acid were similar and non-additive to those of dibutyryl-cAMP, forskolin and glucagon. Results indicate that carnitine palmitoyltransferase I activity may be controlled by a mechanism of phosphorylation/dephosphorylation.