Palmitoylcarnitine is synthesized through the action of palmitoylcarnitine transferase I - an enzyme specifically inhibited by etomoxir. An increase of the intracellular content of palmitoylcarnitine in neuroblastoma NB-2a cells after administration of carnitine was correlated with an inhibition of cell proliferation and a concomitant promotion of differentiation processes. The activity of protein kinase C was measured in vivo, with cells permeabilized through the use of streptolysin O and a peptide substrate. Palmitoylcarnitine inhibited the phorbol ester stimulated reaction of the peptide phosphorylation in a concentration dependent way. The degree of protein kinase C inhibition was correlated with intracellular increase of the palmitoylcarnitine content, pointing to this compound as a natural modulator of protein kinase C activity.