Central effects of intravenously (i.v.)-administered iohexol were compared with those of iopamidol in a series of tests. Mannitol was used as a reference. As assayed by the primary screening test based on Irwin's method, i.v. administration of mannitol resulted in a score of 0 in ddY mice and a score of 0.6 in ICR mice in the startle response. These results were not different from the data of both iohexol and iopamidol. Iopamidol at a dose of 1750 mgI/kg produced an inhibitory effect on the spontaneous locomotor activity. Iohexol at a dose of 7000 mgI/kg potentiated the duration of thiopental-induced narcosis. Hypothermia was caused by high doses of both iohexol and iopamidol. Electric stimulus increased the mortality of mice pretreated with high doses of iohexol and iopamidol. Both drugs had no notable activities in the anticonvulsant, electroencephalic, muscle relaxant and antinociceptive tests. These results indicate that both iohexol and iopamidol do not necessarily possess a similar pharmacological action. Judging from the LD50 of approximately 15000 mgI/kg for both drugs, they seem unlikely to have a specific pharmacological action on the central nervous system.