Intracerebroventricular (i.c.v.) administration of somatostatin or SMS 201-995 induces excessive grooming behavior in rats. The grooming inducing effect of somatostatin is rather weak, as doses of 300 ng or less did not result in increased total grooming scores. In contrast a dose of 10 ng SMS 201-995 already significantly increased the total grooming scores. However, doses of 100 ng and more did not further increase the total grooming scores reached with a 50 ng dose of this peptide. Systemic administration of SMS 201-995 in doses up to 900 ~tg did not result in excessive grooming behavior. Fhe patterns of excessive grooming induced by i.c.v. SMS 201-995 and somatostatin were characterized by a predominant display of scratching. Since peptide-induced scratching is mainly due to activation of opiate receptor systems it is suggested that opiate receptors are involved in the behavioral response to SMS 201-995 and somatostatin administration. This suggestion is further supported by the suppressive effect of naloxone on excessive grooming induced by these peptides. Haloperidol and neurotensin also suppress the excessive grooming induced by somatostatin but not that induced by SMS 201-995. Finally, tolerance developed to the grooming-inducing effect of SMS 201-995 and somatostatin. In addition there was cross tolerance between somatostatin and SMS 201-995.Somatostatin; SMS 201-995; Grooming behavior