Rats were tested in a large, open field with 16 small holes in the floor. Locomotion, exploration, rearing, and grooming were measured during each 10-min testing session. When tested after injection of intraventricular saline, locomotion, exploration, and rearing were the most frequently observed behaviors. Grooming occurred only infrequently. When tested after intraventricular injection of ACTH 1–24, locomotion, exploration, and rearing were greatly reduced, while grooming was markedly increased. These results indicate that the induction of excessive grooming by ACTH 1–24 is not restricted to the small testing chambers usually used in such experiments and that the effect of the ACTH fragment is specific to grooming. Its effect is not an enhancement of the most prominent response being made in a particular situation.