Total hepatic blood flow and ammonia tolerance test have been measured in dogs before and after portacaval shunt. In spite of a very significant decrease of more than 50% in hepatic blood flow, the hepatic extraction efficiency remained normal. A good correlation was observed between maximal blood ammonia level during tolerance test and total hepatic blood flow. No correlation was found between blood ammonia level and hepatic extraction efficiency. In shunted dogs, the hyperammonemia during the ammonia tolerance test seemed to be determinated by the hepatic blood flow variation more than by the liver parenchymal damage.