The experiments were carried out during the period 2007-2008 in the experimental fields of the Agricultural University – Plovdiv. The aim of the study was to investigate the physiological response of barley and oats to provocatively high doses of some herbicides. Phytotoxic effects can sometimes occur without any outward symptoms. In this case, the presence or absence of an adverse effect can be studied by physiological tests on the photosynthetic and enzymatic activity of the plants, but as a rule such studies are relatively few in number. More often the presence of a negative effect caused by herbicides is judged by a decrease in the yield and quality of crop production, vitality and development of seeds and planting material, taste characteristics, etc.