Hepatic function tests were performed on 48 dogs that had been given primidone, phenytoin, or a combination of anticonvulsant drugs for 6 months or longer. Except for histories of seizures, 44 of the dogs were healthy at the time the tests were performed. Abnormal test results were observed most frequently in dogs given only primidone and in dogs given combinations of anticonvulsant drugs. The test results that were abnormal most often were those for alanine transaminase and alkaline phosphatase activities, and sulfobromophthalein excretion. The dosage of anticonvulsant drug was found to modify certain test results. Statistically significant positive correlations were found between the dosage of primidone and serum alanine transaminase activity and between the dosage of phenytoin and serum alkaline phosphatase activity. Four of the dogs were examined because of signs of weakness and anorexia and 2 also had ascites. Three of the 4 dogs were euthanatized 2 to 49 days after admission with clinical signs compatible with hepatic failure, and cirrhosis of the liver was confirmed at necropsy. The fourth dog died at home and was not necropsied. Four of the remaining 44 dogs that apparently were healthy at the time of examination had abnormalities in hepatic biochemical test results that were comparable with those in the 4 dogs with clinical illness. We concluded that, although results of hepatic biochemical tests frequently may be abnormal in dogs given anticonvulsant drugs long-term, severe hepatic injury is observed less often.(ABSTRACT TRUNCATED AT 250 WORDS)