The influence of lysine vasopressin (LVP) andACTH4–10 treatment on avoidance acquisition in a shuttle-box was studied in hypophysectomized rats. Both peptides restored the impaired avoidance acquisition of such animals. Cessation of treatment resulted in a progressive decrease of avoidance performance of ACTH4–10 treated rats, despite shock reinforcement if no avoidance occurred. In contrast, cessation of LVP administration did not affect subsequent avoidance behavior. Accordingly, ACTH analogues have a short-term and LVP a long-term effect on avoidance acquisition. It is suggested that ACTH analogues restore fear motivation, while LVP is involved in the consolidation of aversively motivated learning processes.