The effect of peripherally administered cholecystokinin-octapeptide (CCK8) was tested on signaled-avoidance behavior, apomorphine-induced stereotypy, and catalepsy. Rats were trained to avoid shock in a signaled shuttle-box avoidance task, and then given CCK8, tetragastrin, or haloperidol. CCK8 (20-3840 micrograms/kg i.p.) reduced avoidance in a dose-dependent manner. The impairment at maximal dose levels was approximately 25% from its predrug level compared to 50% with haloperidol (100 micrograms/kg i.p.). Combined injection of CCK8 (320 micrograms/kg i.p.) and haloperidol (75 micrograms/kg i.p.) reduced avoidance significantly more than either drug alone. The effects of CCK8 were relatively brief in that active avoidance was impaired if CCK8 was injected up to 15 min before the avoidance test. CCK8 also facilitated the extinction of avoidance. Tetragastrin (177 micrograms/kg i.p.), which elicits gastrointestinal effects resembling CCK8, did not affect avoidance. In addition, CCK8 diminished apomorphine-induced stereotypy up to 5 min after CCK8 injection. CCK8 (20-1280 micrograms/kg i.p. and s.c.) failed to produce catalepsy in vertical grip tests. These data suggest that peripherally administered CCK8 has sedative and certain neuroleptic-like effects on behavior.