Inherited portosystemic shunts occur in 2 to 3 per cent of Irish wolfhounds and are associated with high venous ammonia concentrations and signs of hepatic encephalopathy. Moreover, the vast majority of Irish wolfhound pups without signs of hepatic encephalopathy have moderate hyperammonaemia. The aim of this study was to investigate whether the increased ammonia levels in these clinically healthy dogs are caused by low-grade portosystemic shunting, and whether the hyperammonaemia persists in adulthood. The fasting venous ammonia concentration and the fraction of portal blood by-passing the liver, expressed as the shunt index (SI) were measured in 42 Irish wolfhound pups, and the dogs with high SI values were examined post mortem. The ammonia concentration was also measured in 25 adult Irish wolfhounds in which it had been measured when they were seven to eight weeks old. Eleven of the 42 pups had a portosystemic shunt, as evidenced by a high SI (mean 0.82, range 0.12 to 1.00, normal range 0.01 to 0.05) and by post mortem examination. Their mean ammonia concentration was 249 mumol/litre (range 121 to 350). The 31 pups with a normal SI (mean 0.025, range 0.00 to 0.05) had a mean ammonia concentration of 93 mumol/litre (range 51 to 125). In the 25 dogs in which the ammonia concentration was measured twice, the mean concentration at seven to eight weeks of age was 77 mumol/litre (range 47 to 115) and in the adults it was 17 mumol/litre (range 6 to 27) at a mean age of 3.1 years (range 1.0 to 8.9). These results show that Irish wolfhounds with ammonia concentrations > 125 mumol/litre had a portosystemic shunt, whereas the hyperammonaemia in dogs with ammonia concentrations < 120 mumol/litre was transient and of metabolic origin.