A photoelectric plethysmograph was newly devised using a commercial photo-reflector (Hamamatsu Photonics P2826 type) that is a photoelectric device with a light-emitting diode (LED) and a phototransistor (Ptr) combined together. Having fit the photoelectric plethysmograph in the least volume (4 x 4 x 2 mm), we were able to make it inexpensive and compact in the size which permitted us to attach to every place of bodily skin surface under consideration. Compared to a conventional tungsten lamp, the LED has an electric characteristic of a narrow spectral distribution in the infrared range, the characteristic of which makes it possible to take photometric measurements of skin tissue independently of the interference of visible light. Further, the LED has a large share of the spectral distribution with the Ptr output. The fact that the LED produces cool light means that we can avoid the problem of a local vasomotor reflex as an artifact due to heat generation caused by the light source put on some skin surface. Another advantage the Ptr has is in that it is relatively free from both ambient air temperature and prior light history effect. These considerations proved the photo-reflector plethysmograph to be superior to the bulky miniature tungsten lamp-photoconductive cell combination generally employed in photoplethysmo-graphy : Thus, the photo-reflector plethysmograph is a useful psychophysiological device to meet the transducer criteria necessary for quantitative photoplethysmography.