Post-prandial serum bile acid concentrations were measured in 200 Maltese dogs in an attempt to identify those with subclinical portosystemic shunts. Five of these were later shown to have hepatic pathology or abnormal liver function. In the other 195 Maltese, bile acid concentrations ranged from 1 to 362 mumol.L-1 (mean +/- SD, 70 +/- 50 mumol.L-1; median, 65.0 mumol.L-1). Of these, 79% were above the reference range (0 to 31 mumol.L-1) established from 23 mixed-breed control dogs. It was therefore not possible to determine the prevalence of subclinical portosystemic shunts on the basis of bile acid determinations. Further investigation of liver function was performed to investigate why bile acid concentrations were increased in these dogs. Rectal ammonia tolerance tests were normal in 102 of 106 Maltese tested and liver samples (11 dogs) and plasma biochemistry profiles (9 dogs) demonstrated no significant hepatic disease or dysfunction. Of 2 Maltese with hyperammonaemia after administration of ammonium chloride, one had a large congenital portosystemic shunt that was confirmed at surgery. In the other there were no macroscopic portosystemic communications, but a liver biopsy showed histological changes consistent with microscopic portovascular dysplasia. Total serum bile acid concentrations were consistently lower when assessed by high-performance liquid chromatography than by an enzymatic spectrophotometric method. This discrepancy was substantially larger in Maltese than in control dogs, suggesting the presence of an additional reacting substance in the serum of Maltese dogs.