The principles and application prospects of a set-up for simultaneous measurements of oxygen evolution and fluorescence in intact leaves on a fast (10 ms) time scale are descríbed. The oxygen evolution is measured with a photoacoustic technique. The method shows among other things (a) a direct deteimination of the intrinsic yield of 'open' reaction centres (RCs) of photosystem 2 (PS 2), and (b) ihe involvement of a 20-30 % firaction of PS 2 RCs with an apparent low oxygen yield in the light-adapted leaves. Double-flashPS 15 measurements in the light- and dark-adapted leaves have substantiated the absence of inactive RCs in leaves that were kept in the dark for more than 12 h.