Possible mechanisms of resistance to the photosystem II (PSII)-inhibiting herbicide chlorotoluron (3-(3-chioro,p-tolyl)-1,1-dimethylurea) were examined in two resistant biotypes (Peldon Al and Lines El) of Alopecurus myosuroides from the United Kingdom. The resistance indices (the ratio of ED,, values relative to a susceptible standard, Rothamsted) were found to be 28 for Peldon Al and 2.6 for Lines El biotypes, based on whole plant responses. There were no significant differences between resistant and susceptible biotypes in rates of herbicide uptake and translocation. Similarly, the herbicide target site, PSII, was found to be equally sensitive to inhibition by chlorotoluron in all biotypes, indicating changes at the herbicide active site were not responsible for resistance. Both resistant biotypes had a markedly enhanced ability to metabolize chlorotoluron. Following a 24-hr herbicide pulse, 81% of chlorotoluron was metabolized by resistant Peldon Al compared with 37% by Lines El and 6% by susceptible Rothamsted. Twenty-four hours later, 94, 48, and 14% of the chlorotoluron was metabolized by Peldon Al, Lines El, and Rothamsted, respectively. Chlorotoluron metabolism was decreased, and herbicide phytotoxicity correspondingly increased by treatment in the presence of the cytockrome P450 inhibitor 1-aminobenzoiriazole, suggesting that increased activity of cytochrome P450 monooxygenses may be responsible for herbicide degradation and, therefore, herbicide resistance in two resistant biotypes. (C) 1995 Academic Press. Inc.