Male F344 rats were exposed to hydrazobenzene (HZB) by dietary feed at concentrations of 0, 5, 20, 80, 200, or 300 ppm for 5 days, 2 weeks, 4 weeks, or 13 weeks duration. End points evaluated included clinical observations, body weights, liver weights, serum chemistry, blood HZB, gross pathology, and liver histopathology. There were no HZB exposure-related clinical signs of toxicity. During study weeks 8 through 13, body weight means in rats of the 300 ppm group were 6% lower compared to control rat means. Serum alkaline phosphatase concentrations were decreased in rats of the 300 ppm group at all time points. Relative (to body weight) liver weight increases were observed in rats of the 200 and 300 ppm groups following 5 days (300 ppm only), 2 weeks, 4 weeks, and 13 weeks of exposure. Following 13 weeks of exposure, microscopic findings in the liver were observed only in rats of the 200 and 300 ppm groups and consisted of hypertrophy, macrovesiculation, eosinophilic granular cytoplasm, and bile duct duplication. Blood HZB concentrations ranged from 0.002 to 0.006 µg/mL in rats of the 200 or 300 ppm groups. A no observed effect level of 80 ppm (4.80 mg/kg per d) was selected based on the observation of microscopic hepatocyte alterations at ≥200 ppm HZB.