Many animal studies show an enhancing effect of vasopressin (VP) on memory, but not all human studies could confirm this finding. This study examined the influence of post-learning administration of VP (40 IU, intranasally) on the consolidation of declarative memories in healthy humans during different intervals of sleep and waking. We could not find any effect of VP on memory consolidation, but EEG activity indicated a significant arousing influence of VP. Results suggest that if VP affects memory function it might do so primarily at the stage of encoding of the materials to be learned but it leaves unaffected processes of consolidation.