A vasopressin V1B receptor antagonist has been shown to exhibit anxiolytic effects in a variety of animal models of anxiety. In the present study, we examined the involvement of the pituitary in the anxiolytic effects of a vasopressin V1B receptor antagonist by conducting a social interaction test in rats. In the sham-operated rats, both the vasopressin V1B receptor antagonist SSR149415 and the benzodiazepine chlordiazepoxide significantly increased the social behavior of a pair of unfamiliar rats, and the blood adrenocorticotropic hormone levels were markedly increased during the social interaction test. Hypophysectomy also increased the length of time that the animals engaged in social behavior to the same extent as that observed after treatment of the sham-operated rats with anxiolytics. However, while chlordiazepoxide further increased the duration of social interaction in the hypophysectomized rats, the anxiolytic effects of SSR149415 was no longer observed in these animals. These results suggest that the anxiolytic effects of the vasopressin V1B receptor antagonist in the social interaction test are mediated through blockade of the vasopressin V1B receptor in the pituitary.