Calcitonin-producing cells (C-cells) are endocrine cells derived from the neural crest. We examined the effects of three types of protein kinase inhibitors on the induction of neuronal phenotypes in the rat thyroid C-cells in vitro. In a primary culture of 16-day-old fetal rat thyroid glands, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7, 25-75 μM) induced both process extrusion and expression of highly polysialylated neural cell adhesion molecule (NCAM) in the C-cells. These effects of H-7 were completely prevented by okadaic acid, a potent protein phosphatase inhibitor. In contrast to H-7, selective inhibitors for cyclic nucleotide-dependent protein kinases such as N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride (HA1004, 25-200 μM) and N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89, 0.25-20 μM) failed to induce process extrusion or the expression of highly polysialylated NCAM in fetal rat C-cells. In cultured C-cells of adult origin, H-7 failed to induce marked process elongation or the expression of highly polysialylated NCAM. These results suggest that the morphological plasticity of the fetal C-cells depends upon the degree of phosphorylation of some proteins, and that the plasticity of adult C-cells are more restricted than that of fetal origin.