Naltrexone, a specific opiate receptor antagonist, is used clinically in the treatment of heroin addiction and more recently, for the treatment of dyskinesia associated with Huntington's disease (HD). Naltrexone may act as a potential hepatotoxin, as reflected in the elevation of transaminase levels. However, one study concluded that, for a brief treatment period of 12 weeks, there is no contraindication to naltrexone treatment based solely on increased hepatic enzyme values. This study monitored liver transaminase levels, in ten HD patients receiving daily doses, between 50 mg/day and 300 mg/day, of naltrexone for periods of 10 to 36 months. Serum glutamic oxalacetic transaminase (SGOT) and serum glutamic pyruvic transaminase (SGPT) levels were obtained before treatment and at intervals of 1 to 4 months during treatment. Only one of the ten patients treated with naltrexone had increased levels of both SGOT and SGPT, whereas one other patient showed elevated levels of SGPT. These elevations, which initially appeared dose related decreased to normal limits with continued treatment. Because many of the patients were receiving other medications, a combination of drugs with naltrexone may contribute to the increased transaminase levels seen in two of the patients. In summary, chronic administration of naltrexone in doses up to 300 mg/day for periods up to 36 months does not significantly change hepatic function, as measured by SGOT and SGPT levels.