[Review with 34 references] The rheoencephalogram is the recording of the variations in impedance of the head to the passage of a small alternating current. This variation is due to the pulsatile flow of blood, the blood having electrical conductance characteristics different than the other tissues of the head. In appearance, the recording resembles that of a pressure pulse or volume pulse curve. The form of the R.E.G. curve varies with pathology. Decreases in amplitude occur when there is interference with the rate and acceleration with which blood enters the head; when there is a mechanical interposition of some substance (e.g., subdural hematoma) between the electrodes and the area of the brain being recorded. Rise time, expressed as a percentage of the overall wave period, is increased in those conditions which interfere with the ready expansibility of the cerebral vascular tree and/or blood acceleration. Similar conditions also affect the appearance of the "dicrotic notch" or secondary oscillations with a resultant smoothing out of the downward limb of the curve. Those conditions which appear to interfere with the ready outflow of blood from the brain appear to result in a raising of the descending limb of the R.E.G., so that it becomes convex upwards. There is some indication that the time from the QRS of the EKG to the initiation of the R.E.G. is shortened in those conditions in which there is sclerosis of the vessels conducting the blood to the head. Some reservation is indicated in the evaluation of the reported investigations, as some of these show an acceptance of factors which are open to dispute.