Norepinephrine (NE) and dopamine (DA) concentrations in 17 individual hypothalamic nuclei and 3 other brain regions were measured in rats, acutely or repeatedly stressed by immobilization, using a microdissection technique and a radioisotopic-enzymatic assay. Following the first 20 min immobilization (IMO) a significant NE decrease in the ventromedial (NVM) and supraoptic (NSO) nuclei and a DA decrease in the arcuate nucleus (NA) as well as NE and DA increase in the dorsomedial nucleus (NDM) were seen. Repeated IMO (40 times) produced a NE increase in the NVM, NDM, NSO paraventricular nucleus (NPV) and median eminence (ME), and a DA increase in the NDM and NPV. Changes of NE and DA concentration found in some individual hypothalamic nuclei under the influence of stress indicate that catecholamines (CAs), particularly in the medial basal hypothalamus, could be involved in the regulation of some neuroendocrine processes which are being activated during stress, especially ACTH release.