Vasopressin may regulate the regional cerebral blood flow (rCBF) via two balancing effects: increased flow from the vessels dilated by nitric oxide from the endothelium, and decreased flow from the vessels contracted by direct stimulation of smooth muscle. The effect on the rCBF in anesthetized dogs following the intracisternal or intraarterial administration of vasopressin was investigated by laser flowmetry with the device placed on the dura over the parietal cortex. The intracisternal injection of 1 nmol vasopressin significantly increased the rCBF to 145.3 +/- 27.3% of base line. In contrast, the intravertebral arterial injection of vasopressin had no significant effect on the rCBF. This can be explained by a difference in the affected vasculature; mainly large vessels in the subarachnoid space vs. whole vascular system supplied by the vertebral artery. The intracisternal injection of 10 mumol of NG-monomethyl-L-arginine (L-NMMA) reduced the rCBF; pretreatment with this agent significantly suppressed the elevation in rCBF induced by vasopressin. The intraarterial injection of L-NMMA reduced the rCBF more than its intracisternal administration. It also suppressed the rCBF induced by vasopressin.