Summary:Inhibition of photosynthesis by metamitron in the rooting medium and its subsequent recovery after transfer of the roots to herbicide‐free nutrient solution was measured in eight plant species.Fast and complete recovery within a few hours after treatment showed that metamitron, once absorbed, was rapidly and completely inactivated in the leaves of sugar beet (Beta vulgarisL.). Inactivation in perennial ryegrass (Lolium perenneL.) was slower and incomplete. It was low inEchinochloa crusgalli(L.) P.B.,Amaranthus retroflexusL.,Alopecurus myosuroidesHuds. and bean (Phaseolus vulgarisL.), and undetectable in maize (Zea maysL.) andPortulaca oleraceaL.From the transpiration rate and the concentration of metamitron in the nutrient solution that just did not cause inhibition of photosynthesis in sugar beet, uptake and inactivation rates per unit leaf area were calculated to be at least 18·5 ng/cm2/h. The same external concentration markedly depressed photosynthesis in the other more susceptible species.After leaf sprays sugar beet plants gradually resumed the normal rate of photosynthesis, but bean plants did not recov