Radiographic quality as well as adverse effects of intrathecal metrizamide instillation was prospectively investigated in thirty-three clinical cases admitted to the department of neurosurgery, University of Tokyo Hospital, and Kantoh Teishin Hospital. Metrizamide CT cisternography was performed in fifteen cases using in most cases 10 ml of 170 mg I/ml solution through lumbar route. Eleven cases exhibited "normal" pattern CSF circulation and the remaining four, "delayed" pattern. Eight cases (53%) experienced headache, nausea, and/or vomiting several hours after the instillation. All of these belong to the "normal" pattern group. Four cases of "normal" pattern received electroencephalographic examinations before and after metrizamide instillation. Three revealed appearance of negative spike and slow wave burst or sharp waves one to twenty-four hours after the instillation, along with penetration of metrizamide into brain parenchyma. Diagnostic quality was interpreted as "good" in eleven cases. Small acoustic neurinoma, pituitary adenoma, arachnoid cyst, and subdural hygroma were diagnosed among others. Metrizamide ventriculography was done in four cases. No untoward effect of significance was attributed to metrizamide per se. Cervical myelograpy and/or CT myelography was done in fourteen cases using, in most cases, 10 ml of metrizamide 170 mgI/ml. Polytome tomography with metrizamide instillation through lateral cervical puncture was highly diagnostic, whereas, ordinary X-ray with lumbar instillation yielded less satisfactory results. CT myelography in cases of subarachnoid block required good consideration on instillation site and positioning of the patient. Six cases (50%) among twelve cases where metrizamide had run into the cranial cavity experienced headache, nausea, and/or vomiting to a lesser degree than those of cisterno graphy. Metrizamide is the first contrast agent ever made which can be safely introduced into human subarachnoid space, if administered judiciously, nervous. However, metrizamide is weakly toxic to central system and provokes minor untoward effects as well as electroencephalographic abnormalities and, sometimes, clinical convulsive seizure. It would be wiser to restrict the dosage of metrizamide in cisternographic study, expecially in cases of "normal" pattern CSF circulation, to 1.2 gI or 7 ml of 170 mg I/ml solution. Routine use of X-ray cisternography should thus be discouraged because it needs higher concentration of metrizamide in the intracranial cisterns.