A 55-year patient with obsessive-compulsive disorder showed clear improvement during 4 weeks of treatment with intranasal oxytocin compared to 4 weeks of intranasal placebo. This improvement was concurrent with the development of severe memory disturbances, supporting the amnestic properties of the peptide. However, the patient also developed psychotic symptoms and a marked decrease in plasma sodium and osmolality, which may have masked the obsessive symptomatology. This case highlights the need for careful monitoring in long-term oxytocin therapy.