Hydrogen (H,) evolution rates were measured by the gas chromatographic technique upon illumination of different subchloroplast preparations of higher plants without exogenous hydrogenase under anaerobic conditions. Subchloroplast preparations enriched in photosystem II (PS II) in the presence of an electron donor TMPD (N,N,N,W-tetramethyl-p-phenylenediamine)are shown to have higher Hz-evolution rates (up to 30 nmol/mg Chl per h) than preparations enriched in PS I under the same conditions. The data on the suppression of & evolution by well-known inhibitors of PS II (dinoseb,atrazine) prove that the Hz photoproduction is sensitized by PS II reaction centers.