Field and laboratory experiments with undisturbed soil columns were performed for assessing the mobility and persistence or chloridazon and lenacil in a clayey soil in the marshes of Lebrija, southwest Spain. in the laboratory we tried to evaluate the herbicides fate when applied with doses greater than normal, as it happens by overlap when spraying the herbicides. Thus, the herbicides doses in the field experiments were similar to those applied by the growers in the area, while the doses applied to the soil columns were four times greater. Apart from that, in the field experiments the herbicides were incorporated into the soil just after the application, by sprinkling irrigation, as it is usually made by the growers. Sometimes, however, there is a delay in irrigating after herbicide application, which may favour adsorption by the soil. To evaluate how this can influence the fate of the studied herbicides in the soil, in the soil columns water was applied for the first time one week after applying the herbicides. Both herbicides showed a low mobility, being the amounts of residues found in the top 0.05 m of soil several times greater than deeper in the soil. Negligible amounts of chloridazon were found at about 4 weeks after application, both in the field and in the soil columns. Lenacil showed a greater persistence, taking about 7 weeks after application for the herbicide to dissipate. The half-life values calculated from the field samples were 11 d for chloridazon and 16 d for lenacil. Calculations from the data of the soil column samples yielded half-life values of 4 d for chloridazon and 14 d for lenacil. The air temperature recorded in the laboratory during the experiments (18±2°C) was greater than in the field (average maximum temperature= 17.2°C; average minimum temperature = R.7°C; minimum temperature = 1.5°C), which may explain the quicker dissipation of the herbicides in the soil columns. Also the higher herbicide concentration in column experiments could to have enhanced biodegradation process. For the studied conditions, neither chloridazon nor lenacil represent a serious risk of groundwater contamination . The values of the coefficient of variation of the herbicide residues in soil samples from the columns were similar to those from the field, suggesting that the technique for herbicide application in the soil columns should be improved to decrease variability.