Microinjection of arginine vasopressin into the lateral septum and bed nucleus of the stria terminalis of male hamsters stimulates intense flank marking and flank gland grooming, while microinjections of vasopressin in sites immediately adjacent to these areas or in the lateral ventricle are ineffective. Microinjections of oxytocin, angiotensin II and the behaviorally active C-terminal fragment of vasopressin, metabolite neuropeptide, by comparison, do not stimulate flank marking. Effective sites for vasopressin injection are clearly superimposable upon autoradiographically defined sites of high V1-receptor density. Furthermore, vasopressin-sensitive neurons in the lateral septum and bed nucleus of the stria terminalis are necessary for the expression of naturally elicited flank marking since the microinjection of a V1-receptor antagonist into these sites was able to temporarily block flank marking triggered by odors from conspecifics.