Organic cation transporters (OCTs) are multispecific, bidirectional, corticosterone-sensitive transporters, primarily studied in the periphery, substrates of which include serotonin (5-HT), noradrenaline (NA), and dopamine (DA). Our previous studies demonstrated OCT3 expression in the rat dorsomedial hypothalamus (Gasser et al. 2006), a region that accumulates 5-HT, NA and DA in response to acute stress or corticosterone administration (Lowry et al. 2001, 2003). These studies also demonstrated bidirectional, corticosterone-sensitive transport of OCT substrates by medial hypothalamic tissue in vitro (Gasser et al. 2006). We have hypothesized that inhibition of OCT-mediated monoamine transport represents a general mechanism by which corticosterone rapidly modulates physiological and behavioural responses. In order to begin to understand the importance of OCT3 in the regulation of monoaminergic neurotransmission in hypothalamic and extrahypothalamic brain regions, we used immunohistochemical methods to characterize the distribution of OCT3-like immunoreactive (OCT3-ir) cells in the rat brain. Three adult male Sprague-Dawley rats were terminally anesthetized and perfused transcardially with 4% paraformaldehyde in 0.1 M phosphate buffer. Brains were removed, postfixed, cryoprotected, and frozen. Frozen sections of 25 µm thickness were made in the coronal plane throughout the rostrocaudal extent of the brain. Free-floating sections were incubated for 12-36 hours in affinity-purified rabbit antiserum directed against rat OCT3. OCT3-ir cells were detected in the ependymal linings of the olfactory, lateral, third and fourth ventricles, and the cerebral aqueduct. In addition, OCT3-ir glial-like (6-8 µm diameter) and neuronal-like (15-20 µm diameter) cells were were detected. OCT3-ir glial-like cells were widely distributed throughout the brain. Particularly high densities of OCT3-ir glial-like cells were observed in areas involved in the modulation of circadian and stress-induced hypothalamic-pituitary-adrenal (HPA) axis activity. These areas included the suprachiasmatic nucleus, bed nucleus of the stria terminalis, dorsomedial hypothalamic nucleus, medial mammillary nucleus, ventrolateral septum, subiculum, and posterior hypothalamic nucleus. OCT3-ir neuronal-like cells were observed in relatively few areas including several areas involved in modulating HPA axis activity such as the bed nucleus of the stria terminalis, medial amygdaloid nucleus, and hippocampus. These data support the hypothesis that corticosterone rapidly alters monoaminergic neurotransmission in neural systems regulating circadian and stress-induced HPA axis activity.