Tolvaptan (INN), also known as OPC-41061, is a selective, competitive arginine vasopressin receptor 2 antagonist under investigation as a potential treatment for fluid retention and hyponatremia (low blood sodium levels) associated with congestive heart failure. Tolvaptan is also in fast-track clinical trials for treatment of polycystic kidney disease.

In a 2004 trial, tolvaptan, when administered with traditional diuretics, was noted to increase excretion of excess fluids and improve blood sodium levels in patients with heart failure without producing side effects such as hypotension (low blood pressure) or hypokalemia (decreased blood levels of potassium) and without having an adverse effect on kidney function.