SummaryThe rate of photosynthesis in intact leaves attached to plants of an atrazine‐resistant cultivar ofBrassica napusL. was compared with that in leaves of an atrazine‐susceptible, rather isogenic cultivar, in a number of irradiance series. The resistant cultivar had been obtained from the susceptible cultivar by crossing the resistant biotype ofBrassica campestrisL. with the susceptible cultivar ofB. napus, and using the fourth back‐cross for comparison. At a natural CO2concentration (360 cm3m−3) the rate of photosynthesis of leaves of the atrazine‐resistant cultivar was significantly (17–21%) less than that of leaves of the susceptible cultivar, over the whole range of irradiances up to 250 W m−2At the enriched CO2concentration of 1500 cm−3m−3this reduction in rate