Glomerulopressin is a hormone that increases glomerular filtration rate (GFR) in toads, rats, and dogs. In this study, the effect of glomerulopressin was assayed in two models of acute prerenal failure, by haemorrhage and by partial occlusion of the aortic artery. In animals with 30 min of anuria both mannitol and glomerulopressin increased GFR and urinary flow, but if the rats were anuric for 120 min, only glomerulopressin was effective in restoring GFR and urinary flow. Glomerulopressin also restored GFR and urinary flow in animals that had remained anuric for 24 hours.