Groups of 3 male and 3 female wild-caught cynomolgus monkeys were given iodixanol, a radiographic contrast medium, by intravenous injection at dosage levels of 100, 300 or 1, 000 mgI/kg/day for four weeks to evaluate its toxicity. An extra 2 animals of each sex were given 1, 000 mgI/kg/day for 4 weeks and then retained for 4 weeks without treatment to assess recovery. There were no deaths during the treatment period. Bruising at the injection sites was noted clinically and macroscopically was caused in part by the injection procedure itself and in part by the viscosity of the test formulation and the size of the dose. This finding is therefore, of no toxicological significance. No effects of treatment with iodixanol on body weights, food consumption, laboratory investigations, ophthalmoscopic examinations, organ weights or bone myelograms were noted. Cytoplasmic vacuolation of the kidney proximal tubules was noted with incidence and severity increasing with dosage level among animals given 100, 300 or 1, 000 mgI/kg/day. This effect was noted not to be fully reversible at 1, 000 mgI/kg/day after 4 weeks off-dose although reduced severity was noted and recovery was apparent in some animals.