The effects of catecholamines (CAs) other than their transmitter action were investigated using clonal neuronal cells, ML-DmBG 2-c2, derived from Drosophila in the larval central nervous system (CNS). All catecholamines tested, adrenaline (AD), dopamine (DA), noradrenaline (NA) and isoproterenol (ISO), prevented any increase in the number of cells during 2-to 7-day culture. α-, β-adrenergic and dopaminergic antagonists did not block the effects of CAs on the number of cells. Adrenochrome, a product of the oxidative degradation of AD, also prevented any increase in the number of cells, as AD did. The effect of AD was partially blocked by an antioxdant, dithiothreitol (DTT). These results suggest that the inhibition of the increase in cell numbers by CA might be mediated by CAs themselves and/or oxidative products in the CA metabolic process. It is concluded that CAs inhibit cell proliferation but do not induce cell death in the Drosophila clonal cells. (J Nippon Med Sch 1999 ; 66 : 113-118)