The 26-week oral toxicity of benzalazine (2-hydroxy-5-[(4-carboxyphenyl) azo]benzoic acid, CAS 64896-26-0), a new agent for the treatment of ulcerative colitis and Crohn's disease of the large intestine, was investigated in beagle dogs of both sexes. No change was observed in the 160 mg/kg group. A reduction of the aspartate aminotransferase activity was observed from 800 mg/kg b.w./d onwards. In high-dosed dogs (1600 mg/kg b.w./d) liver weights were increased and substance-related neutral fat disposition in liver cells was observed. The non-toxic dose was 160 mg/kg b.w./d under these experimental conditions.