Temporal changes in catecholamine activity within the paraventricular nucleus (PVN) of the hypothalamus were assessed using in vivo voltammetry after moderate hemorrhage in chloralose-anesthetized cats. Oxidation current was measured with carbon microelectrodes, and the change at +250 mV was used as an estimate of catecholamine activity. The magnitude and directional change in catecholamine activity was assessed for each site during the initial 3 min of blood loss. Of 62 recording sites, 45 sites were located along the rostrocaudal extent of the medial PVN. Fifteen of these 45 sites exhibited an increase, 15 others exhibited a decrease, and the remaining 15 sites exhibited no change in catecholamine activity. The magnitude of change in oxidation current was independent of the magnitude of the decrease in arterial pressure after hemorrhage. Twelve of the 15 sites that exhibited increases in catecholamine activity were located in the caudal PVN along its dorsomedial aspect, whereas the location of sites that exhibited decreases in catecholamine activity were more widely distributed throughout the nucleus. These data are consistent with the hypothesis that an increase in the release of catecholamines within the caudal PVN in response to hemorrhage is facilitatory for the release of adrenocorticotropin and/or vasopressin.