Experiments were carried out on male SD rats anesthetized with urethane (700 mg/kg) and chloralose (35 mg/kg). The results were as follows: (1) The pressor response could be elicited by electrical stimulation of dorsol part of midbrain periaqueductal gray (dPAG) for 5 s every 5 min. The pressor response for each series of dPAG stimulation within 50 min was constant. This pressor response could be decreased by electrolytic cauterization of paraventricular (PVN) area, but unaffected by electrolytic lesion of fornix, anterior hypothalamic area, nucleus dorsomedialis hypothalami and nucleus ventromedicalis hypothalami. (2) Microinjection of an AVP-V1 receptor antagonist (CH2)5 [Tyr(Me)2AVP] (each side 0.1 nmol/0.1 microliter) into the bilateral rostral ventrolateral medulla (rVLM) partially inhibited the pressor response induced by PVN stimulation or by microinjection of D,L-homocysteric acid (DLH) (0.1 mol/0.2 microliter) into the PVN, but the mean artrtial pressure (MBP) and heart rate (HR) did not show any change when an AVP-V1 receptor antagonist or DLH was only used. (3) Microinjection of an AVP-V1 receptor antagonist into the bilateral rVLM partially inhibited the pressor response induced by dPAG stimulation. Therefore it is suggested that the effect of vasopression (AVP) released from PVN on the pressor response induced by dPAG stimulation is partly mediated by activation of AVP-V1 receptor in the rVLM.