Simultaneous measurements of chlorophyll (Chl) fluorescence and photosynthetic O2 evolution {P^ in leaf disks from Vicia faba (cv. Reina Blanca) leaves were doně during the first weeks of growth in order to evaluate the damage caused by methabenzthiazuron (MBT) under field conditions. The plants were treated at preemergence with two doses of MBT (0.25 .and 0,40 g m'^). Measurements were carried out at constant temperature and saturating CO2 concentration. During the first weeks after plant emergence both doses of herbicide were followed by a drop in and in some fluorescence parameters, such as Fy/Fn, and AF/Fn,' ratios, and in photochemical and non-photochemical quenching, qp and q^. At the same time, increases in minimal fluorescence (Fq and Fq') and in maximal (Fj„') and steady-state fluorescence (Fj) were aiso observed on application of saturating pulses. Fluorescence parameters gave a better indication of the damage caused to the photosynthetic systém than the measurements. The linear relationship found between OO2 and the effective quantum yield, AF/Fn,', suggests that the latter is a good measure of the quantum efficiency of photosynthesis in herbicide-treated plants. Moreover, plant tissues detoxified the herbicide and, one month after plant emergence, photosynthesis was fully recovered in herbicide-treated plants; biomass production recovered and even increased at the end of the growth period.