Abstract The development of the hypothalamic catecholaminergic system during ontogenesis in rats has been studied with glyoxylic acid histofluorescent method in vivo and with the isotopic biochemical technique in vitro. It has been demonstrated that at the 15th fetal day the catecholaminergic system was functionally inactive at least in its ability for the uptake and K +-stimulated release of catecholamines. Since the 16th fetal day, hypothalamic neuronal elements gained an ability for synthesis of catecholamines, their specific uptake and K +-evoked release. Over the subsequent two days, the intensity of the fluorescent intraneuronal product rose considerably showing the increase of either synthesis or accumulation of catecholamines. Simultaneously, the value of the uptake and K +-stimulated release of the exogenous radioactively-labelled dopamine increased significantly. The intensity of the fluorescence of the hypothalamic neuronal elements dropped from 20th fetal until the ninth postnatal day, whereas the specific uptake doubled over the same period reaching itsaadult level. By the 21st postnatal day the reaccumulation of the fluorescent product occurred.