Aqueous myelographic contrast media are needed that are less expensive and better tolerated than metrizamide. This study compares two new, nonionic aqueous contrast media, ioglunide and iopamidol, with two previously developed contrast media, metrizamide and iocarmate. Myelography was performed with one of the contrast media in monkeys; four weeks later the degree of arachnoid fibrosis was determined in each animal by myelographic and histologic studies of the dural sac. Iopamidol, ioglunide, and metrizamide produced no more arachnoid changes than were found in the control animals. Iocarmate produced moderate to severe arachnoiditis.