A microphotoelectrochemical cell (80 µ1) was used to study the generation of photocurrents by photosynthetic membranes isolated from spinach chloroplast. Relatively high (µA range) photocurrents are obtained under potentiostatic mode. The generation of photocurrents is related to a Mehler-type reaction where oxygen serves as electron acceptor during illumination of the photosynthetic membranes and further produces hydrogen peroxide which is degraded at the working electrode, or a suitable artificial electron acceptor is used as mediator. Submembrane fractions enriched in photosystem I or photosystem II can also be used as well as immobilized thylakoid membranes with an increased life-time stability. The potential application for monitoring the electron transport rate or for the detection of phytotoxic compounds in contaminated effluents is discussed.