Ceruletide, a cholecystokinin octapeptide-like substance, has been shown to have some effect on tardive dyskinesia. We, too, previously examined the effect of ceruletide on various types of involuntary movement, and found that responders tended to have high plasma homovanillic acid (HVA) levels. It is generally accepted that both central and peripheral sources make a contribution of plasma HVA. In this study, the response of plasma HVA in rabbits to ceruletide was investigated after pretreatment with debrisoquin sulfate, a drug which selectively blocks peripheral HVA production by inhibition of MAO. As a result, 8 and 50 micrograms/kg ceruletide treatment showed a tendency to decrease plasma HVA levels, but showed no significant differences; however, 140 and 200 micrograms/kg ceruletide showed a significant reduction of plasma HVA. These results are important to the understanding of the mechanism of ceruletide's effect on the brain, as well as to predict the effect of ceruletide on involuntary movements.