An integrated modeling system was developed to predict runoff losses of pesticides from agricultural lands. The system is an integration of a mathematical model, a database system, and a geographic information system. Information on soil type, land use, land slope, watershed boundaries, precipitation, pesticide usage, as well as physical and chemical properties of pesticides have been input to a GIS, managed through a database, and used for further modeling studies. The modeling outputs were in turn put into the database, such that runoff patterns along with pesticides losses could be further simulated by using a database management system. The final results could then be visualized through GIS. The developed modeling system was applied to the Kintore Creek Watershed, Ontario, Canada, for simulating losses of atrazine from agricultural lands. A water quality monitoring project was carried out from 1988 to 1992 in the watershed to detect conditions of surface water pollution due to the use of pesticides. The modeling outputs were verified through the monitoring data, demonstrating reasonable prediction accuracy. The result indicated that the model provides an effective means for forecasting pesticide runoff from agriculture lands.