Arginine vasopress in (AVP) has been shown to have significant effects on thermoregulation. AVP-sensitive locus is located in the septal area of the limbic system. The present experiments were undertaken to investigate the effect of microinjecting AVP and AVP-antiserum into the septal area on the firing activity of temperature-sensitive neurons in preoptic-anterior hypothalamus (PO-AH) of rabbit. The results were as follows: (1) The firing rate of PO-AH warm-sensitive and cold-sensitive neurons were respectively increased and decreased by injection of AVP while that of temperature-insensitive neurons was unaffected. (2) Injection of AVP-antiserum decreased the firing rate of PO-AH warm-sensitive neurons but increased the firing rate of cold-sensitive neurons. (3) Injection of AVP increased the thermosensitivity of PO-AH warm-sensitive neurons and decreased the thermosensitivity of cold-sensitive neurons. The modulation action of AVP on PO-AH temperature-sensitive neurons was discussed.