1. The Laruelle method of base-line ventriculography with small quantities of air, 5 c.c. to 10 c.c., is described. Owing to the small quantity of air used there is very little reaction, and encephalography can be done as an out-patient procedure.2. The absence of disturbance to the patient has made it possible to use this method in investigation of conditions such as epilepsy and cerebral trauma.3. This method should be the first line of investigation in any patient where encephalography or ventriculography may be indicated. This will demonstrate whether any useful information is likely to be obtained by encephalography with larger quantities of air, or ventriculography.4. It is not claimed that this method can replace in every case encephalography with larger quantities of air, or ventriculography.5. Cases of cerebral tumours, trauma, and epilepsy, are demonstrated to show the value of the method.6. The method is of value in the treatment of headaches of post-traumatic and indeterminate origin.