A multiresidue analysis method using solid phase extraction (SPE) and gas chromatography coupled to mass spectrometry (GC-MS) was developed to determine 11 pesticides and 5 of their main degradation products in natural waters. Different parameters affecting the efficiency of the SPE process, such as sorbent type, elution solvent and sample volume were optimized. The best results were obtained by preconcentrating 500 mL of water through Oasis HLB polymeric cartridges and eluting them with acetone and acetonitrile. In order to ensure a good quantification, matrix-matched standards were used, obtaining a good linearity in the range of concentrations studied (0.10-1.5 mu g L-1), with recovery values> 60%, precisions <19% and detection limits <0,1 mu L-1. The proposed methodology was used to evaluate the presence of pesticides and their degradation products in samples of surface and groundwater of the Cachapoal river basin, Central Chile, region affected by an intense agricultural activity.