Photoplethysmography (p.h.p.l.) is a noninvasive method of monitoring changes in peripheral blood volume. The method consists of illuminating the skin with a light flux, and detecting transmitted or backscattered light from the vascular bed by a photosensor. Experience and measurements show that the photoconductor (p.h.c.) is among the most sensitive of photosensors, and it is also small, cheap, and does not require high voltages. The p.h.c., however, has the unfavourable characteristics of temperature and light-history dependence, which contribute to drift, and dynamic characteristics that change with different levels of ambient light and temperature level. This paper investigates these unfavourable electrical properties and shows how they can be dealt with so that the p.h.c. can be used with a high level of confidence, and photoplethysmograms obtained with minimum artefact.