Iotrol, a nonionic dimer isotonic with cerebrospinal fluid at 300 mg l/ml, was evaluated against metrizamide, iopamidol, and iohexol by electroencephalography (EEG) in nonanesthetized cynomolgus monkeys. Each of four monkeys was injected intrathecally with 0.5 ml/kg of 300 mg l/ml of each contrast medium. EEG was obtained before, 45 min, and 3 hr postinjection. EEG spikes, motor disturbances, convulsions, and behavior changes were observed. The EEG from C3 channel was also computer-analyzed. Average percentage differences between control and test energy content of the entire EEG and of the EEG spectrum subdivided into five frequency bands were calculated. With metrizamide, three animals suffered seizures, and all were lethargic and irritable. With iopamidol, two animals convulsed; all animals were lethargic. Transient apathy and motor disturbances were observed in the iohexol animals. No motor or behavior changes were produced by iotrol. Metrizamide and iopamidol increased the spectrum energy in all bands. Neither iohexol nor iotrol affected the EEG significantly. On the basis of these findings, iotrol holds promise as a clinical contrast material for the subarachnoid space.