Sprague-Dawley rats given azathioprine in the diet for 3 to 4 weeks developed severe liver damage. Elevations of serum alkaline phosphatase and gamma-glutamyl transpeptidase activities were associated with increased hepatic glucose 6-phosphate dehydrogenase levels and decreased liver glucose 6-phosphatase activities, i.e., conditions which were commonly observed in various hepatotoxin-induced liver injuries. Light and electron microscopic observations revealed centrolobular necrosis with large scars and the proliferation of the mitochondria and rough endoplasmic reticulum. This model could be used to study the mechanisms of azathioprine-induced liver damage and its prevention.