The objective was to quantify leaching of the widely used low-dosage sulfonylurea herbicides rimsulfuron and its primary degradation products IN70941 ([N-(4,6-dimethoxypyrimidin-2-yl)-N-((3-ethylsulfonyl)-2-pyridinyl)urea]) and IN70942 ([N-((3-ethylsulfonyl)-2-pyridinyl)-4,6-dimethoxy-2-pyrimidineamine]) at two sandy research fields in Denmark. Water was sampled monthly from the vadose and groundwater zones at the two sites (Tylstrup and Jyndevad) over a 4-6-year period following application of rimsulfuron. No rimsulfuron was detected in the water samples. At the Jyndevad site, IN70941 was detected in the vadose zone at a depth of 1m for as long as three years in annual average concentrations exceeding the EU limit value for drinking water of 0.1microgL(-1). At the Tylstrup site, IN70941 was detected at a depth of 2m in concentrations just below 0.1microgL(-1). The groundwater concentration of IN70941 occasionally exceeded 0.1microgL(-1) at the Jyndevad site, but is only detected on one occasion (and at a low concentration) at the Tylstrup site. At both sites IN70941 was relatively stable and persisted in the soil water for several years, with relatively little degrading further to IN70942. Thus, the concentration of IN7092 was much lower and apart from four samples from the Jyndevad site, never exceeded 0.1microgL(-1). Nevertheless, our findings show that degradation products of rimsulfuron can leach through sandy soils in relatively high concentrations and could potentially contaminate vulnerable aquatic environments. In view of this risk, IN70941 and IN70942 should be included in pesticide monitoring programmes, and their long-term ecotoxicological effects should be investigated further.