The role of adrenoreceptor stimulation and the peripheral mechanism mediating the increase in mean arterial blood pressure (MAP) and heart rate (HR) during administration of histamine (HA) in the paraventricular nucleus/anterior hypothalamus (PVN/AH) was evaluated in conscious rats. HA administered through microdialysis probes in the PVN/AH region increased MAP (18 +/- 1 mmHg) and HR (81 +/- 10 beats/min). The pressor response was abolished by simultaneous administration of phentolamine (alpha 1- and alpha 2-antagonist) or prazosin (alpha 1-antagonist) but not altered by yohimbine (alpha 2-antagonist). The tachycardia was not effected by any adrenergic antagonist. Furthermore, ganglionic blockade did not reduce the increase in MAP (21 +/- 2 mmHg) during PVN/AH perfusion with HA, while V1-vasopressin receptor blockade abolished the pressor response (4 +/- 2 mmHg). These data suggest that HA administered to the PVN/AH increases blood pressure by local release of norepinephrine and alpha 1-adrenoreceptor stimulation of vasopressin secretion, while the tachycardia is not mediated by alpha-adrenoreceptors.