The objective of the present study was to investigate the in vitro effects of octanoic acid, which accumulates in medium-chain acyl-CoA dehydrogenase (MCAD) deficiency and in Reye syndrome, on key enzyme activities of energy metabolism in the cerebral cortex of young rats. The activities of the respiratory chain complexes I-IV, creatine kinase, and Na+,K(+)-ATPase were evaluated. Octanoic acid did not alter the electron transport chain and creatine kinase activities, but, in contrast, significantly inhibited Na+,K(+)-ATPase activity both in synaptic plasma membranes and in homogenates prepared from cerebral cortex. Furthermore, decanoic acid, which is also increased in MCAD deficiency, and oleic acid strongly reduced Na+,K(+)-ATPase activity, whereas palmitic acid had no effect. We also examined the effects of incubating glutathione and trolox (alpha-tocopherol) alone or with octanoic acid on Na+,K(+)-ATPase activity. Tested compounds did not affect Na+,K(+)-ATPase activity by itself, but prevented the inhibitory effect of octanoic acid. These results suggest that inhibition of Na+,K(+)-ATPase activity by octanoic acid is possibly mediated by oxidation of essential groups of the enzyme. Considering that Na+,K(+)-ATPase is critical for normal brain function, it is feasible that the significant inhibition of this enzyme activity by octanoate and also by decanoate may be related to the neurological dysfunction found in patients affected by MCAD deficiency and Reye syndrome.