Effects of the injection of 0.25 ml of nonionic radiocontrast media (280 nig I/ml) and/or istonic and/or hypertonic mannitol into the subarachnoid space on the low frequencies (2–5 Hz) of the EEG were studied in nonanesthetized, curarized rabbits. Compared to the noninjected controls, mannitol did not cause any significant EEG spectrum shifts. P-297 and iopamidol caused a slight decrease in the energy content of the 2–5-Hz band, while DS-1–132 and the nonionic dimer ZK 32347 caused a moderate increase. Since an increase in the slow-wave component of the EEG indicates a pacemaking activity, while a decrease indicates stimulation or irritation, none of the tested nonionic media appear to be biologically inert. In this test P-297, iopamidol, and DS-1–132 caused fewer disturbances than metrizamide.