Intraventricular injection of ACTH or peptides of the N-terminal part of ACTH in rats induces excessive grooming behavior. The present study deals with the grooming-induction potencies of short peptides sharing sequences with ACTH4–10 (H-Met — Glu — His — Phe — Arg — Trp — Gly-OH). Although ACTH4–10 itself is inactive, ACTH4–7 and [d-Phe7]ACTH4–10 are capable of inducing excessive grooming behavior. From the ineffectiveness of a variety of other oligopeptides within the ACTH4–10 sequence, it was concluded that ACTH4–7 is the shortest sequence to induce this behavioral response as was found in studies on the effect of these peptides on the maintenance of a conditioned avoidance response. The present data underscore the need of detailed information on the configuration of peptides at the CNS receptor site.