The ontogeny of the rat supraoptic (SON) and paraventricular (PVN) nuclei was studied using a combined fluorescence-immunocytochemical technique for the simultaneous localization of catecholamines (CA) and neurophysin (NP). NP neurons and CA varicosities were first detected in the SON and PVN at 1 days postcoitus. The development of NP neurons which included increases in immunoreactivity in both nuclei proceeded through fetal and neonatal stages, approaching maturity by 21-28 days postnatal; the maturation of the PVN lagged behind that of the SON. CA varicosities appear to make contact with NP neurons beginning at 21-22 days postnatal adult-like patterns were established. The prenatal dominance of NP stain relative to CA fluorescence may suggest a possible neurotrophic role for magnocellular neurons and/or their products upon ingrowing noradrenergic axons.