Inclusion Criteria:  Diagnosis of nephrogenic diabetes insipidus (NDI) (congenital, tolvaptan-induced, or lithium-induced). Morning Uosm < 300 mOsm/kg H2O. Participating in tolvaptan arm. Males for NDI. Autosomal Dominant Polycystic Kidney Disease (ADPKD). Lithium-induced NDI. GFR ≥ 30 ml/min. If hypertensive, blood pressure controlled on antihypertensives (< 130/80 mm Hg) at least 30 days before day 1. Capable of providing consent. Capable of providing urine samples as dictated by the protocol.  Exclusion Criteria:  History of acute gout attack in the past 30 days. Uncontrolled hyperuricemia or active gout. Known urinary retention, urinary incontinence or bladder dysfunction. Other significant chronic medical disease (heart failure, diabetes mellitus, liver disease, transient or persistent elevated transaminases. History of hepatotoxicity related to tolvaptan. Allergy to interventional drug (PB). History of persistent hyponatremia.