Prominent features of aging include a decrease of attention abilities and loss of sleep. Although acute effects of vasopressin (VP) on these functions in the elderly remained inconsistent, beneficial effects of the peptide may develop only with subchronic treatment, which so far has not been tested. This pilot study examined the changes in: i) event-related brain potentials (ERPs) during an attention task, ii) mood, and iii) nocturnal sleep in two healthy elderly subjects during a 3-month period of VP treatment (40 IU/day). The period of VP treatment was preceded and followed by 4-week periods of placebo treatment. ERPs and mood were not consistently affected by VP. However, unexpectedly, VP improved sleep by markedly enhancing nocturnal slow-wave sleep (SWS, p < 0.05). These findings indicate that effects of subchronic treatment with VP involve mechanisms different from those mediating acute effects.