Plethysmography involves recording the volume of a body segment in accordance with the amount of blood in the segment over the testing period. A discussion of the principles of the operation of photoelectric plethysmographs focuses on the two basic types; one operates via light transmission, and the other uses light reflection. The author advises that it is possible to create a photoplethysmograph system that can display total volume change in a body segment; however, when this is done, it is difficult to keep the baseline of the tracing centered on the record, because slight displacement of the photoplethysmograph and small volume changes in the underlying tissues will produce large displacements in the baseline of the recording. In addition, the pulsatile changes in amplitude would be small; however, with capacitive coupling, volume shifts do not emerge on the baseline, and the recording appears more stable, so it is possible to increase the sensitivity to make the pulsatile changes clearly visible. The report also discusses the physiological responses that are recordable with the photoplethysmograph. The author advises that in the design of a photoplethysmograph, care must be taken to guarantee that the light source does not produce sufficient local heating to alter the degree of vasodilation or vasoconstriction that existed before the device was applied. To minimize this effect, many instruments use either a small, low wattage bulb operated below its rate voltage or use a light-emitting diode. Just how much heating is permissible to obtain the most useful information for a polygraph examination is not yet known. Still, with a time constant of 2 seconds, polygraph recordings can be obtained and investigated for their value in polygraph examinations. 4 figures and 2 references