Systemic injection or arginine vasopressin (AVP) (1 micrograms/rat) significantly prolonged extinction of a pole-jump, active avoidance response in rats; lateral ventricular injection of 1000-fold less AVP (1 ng/rat) produced similar results. A new AVP analogue, [1-deaminopenicillamine-2-(O-methyl)tyrosine]arginine vasopressin (dPTyr-(Me)AVP), is known to antagonize behavioral and vascular effects of exogenous AVP at molar ratios of 5 : 1. At a dose of 100 micrograms/rat (subcutaneously) dPTyr-(Me)AVP produces, by itself, a behavioral effect opposite to that of exogenous AVP, namely a facilitation of extinction. Injections of dPTyr-(Me)AVP into the lateral ventricle were ineffective except at a dose of 10 micrograms/rat. These results confirm previous reports of the effect of vasopressin on delaying extinction of avoidance behavior, but suggest a site of action distant from the lateral ventricle.