Field-grown plants of spring barley (Hordeum vulgare L. cv. Akcent) in the growth phase 30 DC (beginning of stem extension) were exposed to a one-shot application of a commercial product containing cyanazine (Bladex 50 SC) in two doses, C30 and C60 (30 and 60 mg m-2). The reaction of the plant photosynthetic system was followed non-destructively using chlorophyll fluorescence induction (the O-J-I-P transient) within three weeks after the application in the fifth developed leaf and three further gradually appearing leaves. An immediate response of plants to the application of cyanazine and a regeneration of plants from cyanazine action were detected. The biological (plant dry mass) and crop yield production (the number and mass of grains in a spike) were analyzed in time of full ripeness. The crop yield was lowered by the herbicide effect to the same level for the two doses used. and M. Matoušková, J. Nauš, M. Flašarová.