In vivo microdialysis was used to measure changes in extracellular concentrations of catecholamines and indoleamines in freely moving rats in response to administration of corticotropin-releasing factor (CRF). Dialysis probes were placed stereotaxically in either the medial hypothalamus or the medial prefrontal cortex. We used a repeated-measures design in which each rat received artificial CSF or one dose of CRF 3-4 h apart, and each subject was retested with the same treatments in the reverse order 5-7 days later. With the dialysis probe in the hypothalamus, intracerebroventricular administration of CRF (17 or 330 pmol) dose-dependently increased dialysate concentrations of norepinephrine (NE), dopamine (DA), and all their measurable catabolites except normetanephrine. The effects on NE were substantially greater than those on DA. Dialysate concentrations of serotonin could not be measured reliably, but those of its catabolite, 5-hydroxyindoleacetic acid, were also elevated. Concentrations of NE and DA were elevated within the first one or two (20 min) collection periods, with a peak response at approximately 1-2 h. Dialysate concentrations of catecholamines and metabolites normally returned to baseline within 3 h. Similar data were obtained with dialysis probes in the medial prefrontal cortex after intracerebroventricular administration of 17 or 167 pmol of CRF, except that the increases in DA exceeded those of NE in this region. Intraperitoneal administration of CRF (1 nmol) similarly elevated dialysate concentrations of NE, DA, 5-hydroxyindoleacetic acid, and all catecholamine catabolites except normetanephrine in both medial hypothalamus and medial prefrontal cortex. These results support earlier neurochemical data suggesting that CRF administered both centrally and peripherally stimulates the release of both DA and NE in the brain.