The role of the kallikrein system in the natriuresis of SIADH was studied in 3 patients. Following free water intake, serum Na decreased. Urinary Na excretion, urinary aldosterone excretion and urinary kallikrein-like activity (UĸaV) increased. Significant relationships were observed between UĸaV and urinary aldosterone excretion during the study. Further, there were significant relationships between serum Na and UĸaV, and between uric acid clearance and UĸaV. In 1 patient, who was observed for a longer period, a significant relationship was observed between Na clearance and UĸaV. We conclude that UĸaV is increased during hyponatremia in SIADH by volume expansion. This increase in UKaV may play a role in the natriuresis in