Effects of arginine vasopressin (AVP1-9) and its behaviorally active fragments [Cyt6]AVP5-9 and [Cyt6]AVP5-8 were studied on the retention of one-trial learning passive avoidance behavior in rats. Peptides were microinjected into various limbic and midbrain structures (ventral or dorsal hippocampus or the dorsal raphe nucleus) and were administered either immediately after the learning trial (post-learning treatment) or shortly before the 24 h retention session (pre-retention treatment). Doses for intracerebral microinjections were selected after preliminary experiments with subcutaneous and intracerebroventricular peptide administration. AVP1-9 facilitated passive avoidance behavior when the peptide was microinjected into either brain structure, however, the ventral hippocampus appeared to be the most sensitive. In this limbic region, AVP1-9 facilitated passive avoidance behavior in an amount of 8 pg (bilaterally), both when given as post-learning or pre-retention treatment. [Cyt6]AVP5-9 and [Cyt6]AVP5-8 were more effective than the parent nonapeptide in terms that a lower amount of these peptide fragments facilitated passive avoidance behavior in all brain regions investigated. The ventral hippocampus appeared to be the most sensitive brain site for the behaviorally active vasopressin fragments as well. Following microinjections into the ventral hippocampus, [Cyt6]AVP5-8 was more effective in a post-learning than in a pre-retention treatment schedule. [Cyt6]AVP5-9 on the other hand was more effective when injected shortly before the retention trial. The data indicate that limbic-midbrain structures are sensitive to AVP1-9 and behaviorally active putative metabolites of this neuropeptide. The active fragments selectively influence different phases of information processing upon limbic microinjections.