In vivo microdialysis was used to study the effects of single (2 h) or repeated (2 h for 7 consecutive days) immobilization (IMMO) stress on extracellular fluid concentrations of norepinephrine (NE) and the deaminated metabolites of NE and dopamine, dihydroxyphenylglycol (DHPG) and dihydroxyphenylacetic acid (DOPAC) in the paraventricular nucleus of conscious rats. During IMMO, NE, DHPG, and DOPAC levels increased markedly, with similar peak values and time courses in the repeatedly stressed and previously unstressed groups. NE levels during a 2-h baseline period were lower in the repeatedly stressed group than in the unstressed group (99 +/- 9 pg/ml vs. 167 +/- 13 pg/ml, P less than 0.05), whereas DHPG (1,697 +/- 263 pg/ml vs. 1,424 +/- 194 pg/ml) and DOPAC (5,989 +/- 863 pg/ml vs. 4,428 +/- 1150 pg/ml) levels tended to be higher, so that the NE/DHPG ratio at baseline was significantly lower in the repeatedly stressed group (P less than 0.05). The results indicate that IMMO stress enhances NE release, reuptake, metabolism, and synthesis in the PVN. Repeated exposure to IMMO may decrease the microdialysate NE/DHPG ratio by inhibiting exocytotic release or enhancing neuronal reuptake of NE. In either case, the results suggest that repeated exposure to stress alters the release and disposition of NE in the PVN of conscious animals.