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Runoff events in soybean growing areas of the Pampa s plains cause the mobilizatior of pesticides from agricultural fields into water bodies, contributing to invertebrate, fish and anuran kills (Natale and Ronco 2003; Jergents et al. 2004a; Carriquiriborde et al., personal communication). Cypermethrin, Chlorpyrifos and Endosulfan are commonly used insecticides in soybean crops of the region, though their concentratior levels in water and sediments of related surface water bodies has only recently beer characterized (Peruzzo et al, 2003; Jergents et al. 2004a-b;Marino et al. 2004).Taking into account that the extent ot the crops in the region is about ten million hectares of the genetically modified soybean variety (RR), the aim of the present study is to determine concentration levels of cypermetrin and chlorpyrifos in water and sediments from a first order stream of the Pergamino-Arrecifes system, and in the lower basins of five main tributaries flowing into the Parana river. MATERIALS AND METHODSThe study area took into consideration the assessment ot cypermethrin and chlorpyrifos concentrations at two scale levels of the Pampa Ondulada region. For a detailed scale study, a first order stream of the Pergamino-Arrecifes system was chosen (Figure 1) with its source within crop fields. The sampling campaigns during two consecutive years took into account the time for spraying of pesticides and the rain events immediately following sprays (table 1). Three sampling sites were selected to assess a direct influence of agricultural activity: Site 1, adjacent to field crops; Site 2 at 200 meters down stream, following the first site, adjacent to pasture, and Site 3 down stream next to a marshy sector, 500 meters from the first site. For the extended work scale, the assessment covered the lower basins of five rivers traversing crop field production areas. The water courses flow from the west to the east within the province of Buenos Aires, reaching the Parana river in its middle and low sectors (Manassero et al. 2005). Listed from north to south they are: del Medio stream, and Ramallo, Tala, Arrecifes and Areco rivers. All sampling sites and campaign details for the first order tributary are shown in figure 1 and table 1. Campaign sampling dates for the extended work scale are given in table 2.