Publisher Summary The hypothalamic-neurohypophyseal system possibly makes use of (a) the general circulation for peripheral effects of posterior pituitary hormones; (b) the portal vessel system for the regulation of anterior pituitary function; and (c) the cerebrospinal fluid for CNS activities. Evidence is presented in the chapter that vasopressin and its analogues facilitate the consolidation of learned behavior patterns. Under certain conditions, these peptides facilitate the acquisition of active avoidance behavior and increase the resistance to the extinction of active and passive avoidance behavior and of sexually motivated approach behavior as well. Intraventricular administration of minute amounts of vasopressin analogues facilitates memory consolidation. This supports the idea that the behavioral effect of these polypeptides is centrally mediated. Vasopressin antibodies, which are assumed to neutralize in situ vasopressin released into the cerebrospinal fluid (CSF), prevent memory consolidation. Studies on paradoxical sleep in diabetes insipidus rats reveal disturbances in hippocampal theta frequencies and strengthen the hypothesis that memory consolidation is under the influence of vasopressin analogues. The development of resistance to the analgesic action of narcotic analgesics is facilitated by the administration of vasopressin analogues and markedly retarded in diabetes insipidus rats. These and other results suggest that the memory consolidating effects of vasopressin analogues are of a more general nature.