SummaryThe effect of chlorfenprop‐methyl, flampropisopropyl and benzoylprop‐ethyl on14CO2fixation was followed in wild oat (Avena fatuaL.), barley (Hordeum vulgareL., cv. Ametyst), and wheat (Triticum aestivumL., cv. Mironovská). Experimental plants were exposed to a14CO2‐enriched atmosphere in a special apparatus 2 h, 1, 3, and 9 days after the herbicide treatment.Chlorfenprop‐methyl already inhibited14CO2fixation in wild oat plants 2 h after the treatment.14C‐metabolite transport to the roots was strongly decreased. Both14CO2fixation and14C‐metabolite level in the roots were significantly depressed inA. fatuawhen compared with untreated plants at the last sampling time.14C incorporation into starch was inhibited from the first day after treatment, and on day 9 was lowered more than ten fold in treated plants.Flamprop‐isopropyl inhibited14CO2fixation in wild oat plants from day 3 after treatment, but benzoylprop‐ethyl not until day 9. Both herbicides also decreased14C incorporation into starch inA. fatua.Chlorfenprop‐methyl also slightly decreased14CO2fixation in barley on day 9. However, assimilate transport into the roots and14C incorporation into starch were not affected. Flamprop‐isopropyl inhibited14CO2fixation in barley plants only on the first day after treatment, and assimilate transport was also reduced. By contrast, no differences from untreated plants were found at the end of the experiment. Benzoylprop‐ethyl did not decrease either14CO2 fixation or assimilate transport to the roots in wheat, but it inhibited starch synthesis. Atrazine depressed14CO2fixation in wild oat plants by 91%, in wheat plants by 99% compared with untreated plants. Assimilate transport into the roots was also strongly inhibited. In contrast to atrazine, the effect of chlorfenprop‐methyl, flamprop‐isopropyl, and benzoylprop‐ethyl on CO2fi