A 76-year-old man with small cell lung cancer associated with the syndrome of inappropriate secretion of ADH (SIADH) visited our hospital. The serum Na level was normal on the first visit, but 2 weeks later it decreased to 114 mEq/L with an extremely high plasma vasopressin (VP) level of 1520 pg/ml. Serum Na was normalized after the reduction of the tumor size by chemotherapy, but the plasma VP level remained between 150 to 600 pg/ml. On gel filtration of plasma VP two peaks of immunoreactive VP were eluted at the positions of a larger molecule than authentic VP and authentic VP, and VP in urine gave only one peak compared to that of authentic VP. The dilution curve of plasma VP was almost parallel and that of urine was completely parallel to the standard curve. These findings suggest that a larger VP with low physiological activity was predominantly secreted in the present patient and manifested relatively mild symptoms despite the extremely high plasma VP level.