Groups of 15 rats of each sex were fed Green S at dietary concentrations to provide dose levels of 0 (control), 250, 500 or 1500 mg/kg body weight/day for 13 wk. Additional groups of five animals of each sex were given the same treatments for 2 or 6 wk. There was a marked excretion of green colour in the faeces and some green colouring of the urine, although the latter may have been due to contamination. The males showed increased water and food intakes associated, particularly at the highest dose, with a higher rate of body-weight gain. Haematological examination revealed a transitory mild anaemia at the highest dose level, whilst no findings indicative of a toxic effect were found in the renal concentration tests or the serum analyses. With a dose of 1500 mg Green S/kg a greater proportion of the rats showed higher urinary protein, protein casts, increased caecal weight, thyroid degeneration in female animals and enlargement of the lymph nodes in the intestine wall. The no-effect dose level for Green S in this study was considered to be 500 mg/kg.