Rats tested in a step-through inhibitory avoidance task were administered hypertonic saline (2 ml of 0.25. 0.5, and 1.0 M intraperitoneally) or arginine vasopressin (1.0, 2.0, and 4.0 micrograms) injected subcutaneously (sc) after the training trial where the rats received a mild footshock (0.2 mA, 3 s). Both hypertonic saline and vasopressin produced significant increases in latency to reenter 24 h later. These treatments failed to increase reentry latencies in animals that received the same procedure but no shock. The facilitation of inhibitory avoidance produced by hypertonic saline was reversed by sc administration of 25 micrograms of the vasopressor (V-1) vasopressin antagonist, dPtyr(Me)AVP. The results suggest that the endogenous release of vasopressin can be behaviorally significant in situations of acute homeostatic challenge.