A method of simultaneous recording of the intensity changes in several delayed luminescence (DL) components was ušed to study the effects of HgCl2 on electron transport reaction in intact algal cells. At a low concentration (1 pM), the reagent partially inhibited the electron transport between the acceptors and Qg of photosystem 2 (PS 2), whereas at high concentrations (100 pM) the reagent affected mainly the donor side of PS 2.