Various inhibitors of DNA synthesis induced neurite extension in human neuroblastoma cells. In order to clarify morphology-function relationship in differentiation of neuroblastoma cells, the effect of the replication inhibitors on inducibility of catecholamine synthesis was examined. The reagents alone did not affect production of dopamine and noradrenaline, but joint administration of each inhibitor and sodium butyrate considerably promoted the catecholamine synthesis, without additional change in neurite profile. Although inactive in neurite extension, sodium butyrate was moderately active in catecholamine production. The promoting effect of thymidine (or hydroxyurea) and sodium butyrate was repealed by alpha-amanitin, actinomycin D or cycloheximide.