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The occurrence, distribution, and temporal patterns of pesticide concentrations were studied in the Hudson River Basin during 1994 - 96. This article presents the results of three separate pesticide studies conducted as part of the U.S. Geological Survey (USGS) National Water Quality Assessment (NAWQA) program. Pesticides were found in all three studies, but rarely at concentrations exceeding any U.S. Environmental Protection Agency drinking-water standards. The highest concentrations were detected during and immediately after the first runoff following pesticide applications in the late spring and early summer. The herbicides atrazine and metolachlor were the most commonly detected pesticides and were present in nearly every sample collected from streams draining agricultural areas; they also were detected in many streams draining areas with other land uses. Herbicides were most often detected, and had the highest concentrations, in samples from streams draining agricultural areas, whereas insecticides such as diazinon were most commonly detected, and had the highest concentrations, in samples from streams draining urban areas.