As part of the U.S. Geological Survey's National Water-Quality Assessment Program, the use and occurrence of agricultural pesticides were investigated in four drainage basins--two dominated by irrigated agriculture and two by dryland agriculture--in the Central Columbia Plateau of eastern Washington. For this study, 85 pesticides or pesticide metabolites were selected for analysis from a list of nearly 400 compounds commonly used in the United States. Pesticide-use data included estimates of the total quantity of herbicides, insecticides, and fungicides applied to croplands in each of the four drainage basins and reported times of application for selected pesticides. Pesticide-occurrence data included concentrations of pesticides in samples collected at one surface-water site at or near the outflow of each of the four drainage basins, where surface waters were sampled one to five times a month from March 1993 through May 1994. Of the 85 pesticides or pesticide metabolites targeted for analysis, a total of 45 different compounds were detected in samples from the four sites, ranging in concentration from at or near the limit of detection (as low as 0.001 microgram per liter) to a maximum of 8.1 micrograms per liter. None of the concentrations of pesticides exceeded the U.S. Environmental Protection Agency (USEPA) drinking water standards, but concentrations of five pesticides exceeded the USEPA freshwater-chronic criteria for the protection of aquatic life. Forty-one different pesticides or pesticide metabolites were detected in surface waters sampled at the two sites representing irrigated agriculture drainage basins. The herbicides atrazine, DCPA, and EPTC were detected most frequently at the two sampling sites. Not all pesticides that were applied were detected, however. For example, disulfoton, phorate, and methyl parathion accounted for 15 percent of the insecticides applied in the two irrigated drainage basins, yet none of these pesticides were detected in samples from the two irrigated-agricultural sites. Concentrations of pesticides found in surface waters at the two sites representing irrigated agriculture did not exceed drinking water standards, but some concentrations of the insecticides carbaryl, chlorpyrifos, diazinon, and azinphos-methyl exceeded the freshwater-chronic criteria for the protection of aquatic life. Twenty-three different pesticides or pesticide metabolites were detected in samples from the two sites representing dryland agricultural drainage basins. Herbicides were the type of pesticides most heavily applied in these drainage basins, and the herbicides atrazine, triallate, and simazine were detected most frequently in samples. Some herbicides, for example triallate in the Palouse River drainage basin, were both heavily applied and frequently detected. Others, like atrazine and simazine, were not typically applied to cropland, but were frequently detected in surface-water samples. Several insecticides (Lindane, ethoprop, carbaryl, and azinphos-methyl) were detected in samples from the two sites, although they were not reported as commonly applied to croplands in the dryland agricultural drainage basins. Concentrations of pesticides found in surface waters at the two sites did not exceed drinking water guidelines, but concentrations of the insecticides diazinon and azinphos-methyl and the herbicide triallate exceeded freshwater-chronic criteria for the protection of aquatic life.