Monitoring of aquatic compartments represents an on going challenge for the environmental chemist. Passive sampling has recently been developed as an alternative to grab or average automated sampling, in order to obtain at lower cost, more realistic estimates of the average concentrations of contaminants in surface waters. This technique allows the accumulation of chemicals from large amounts of water, resulting in ultratrace level detection and smoothed integrative sampling over periods ranging from days to months. The lag time is the time required for a compound to reach a steady state flux in the receiving phase. Although it represents relevant information in case of short temporal variations of the organic pollutant concentrations in the aquatic medium, it has been rarely investigated. Passive samplers can monitor a broad range of micropollutants, with various physicochemical properties. Moderately to highly hydrophobic compounds such as polycyclic aromatic hydrocarbons (PAH) and polychlorinated biphenyl congeners (PCB) are sampled by the Semipermeable Membrane Device (SPMD)1, whereas the Polar Compound Integrative Sampler (POCIS) has shown good results for the passive sampling of moderately polar chemicals such as pharmaceuticals compounds and hydrophilic pesticides2-3. Nevertheless, the integrative sampling of hydrophobic pesticides has been poorly reported in the literature. Stir Bar Sorptive Extraction (SBSE) is a solvent free sample preparation technique dedicated to the analysis of moderately hydrophobic compounds in aqueous samples4. The extraction is performed on a stir bar composed of a magnet enclosed in a glass tube coated with a thick film of polydimethylsiloxane (PDMS or silicone). The in situ application of this technique has been reported in the literature only for PAH5. The aim of this study was to develop and calibrate PDMS coated stirs bars as a passive sampling technique for the monitoring of 25 agricultural pesticides in surface waters. A laboratory calibration of the stir bars was realised under controlled temperature and flow velocity conditions in order to determine the sampling rates of the target compounds. A good repeatability was obtained, and the sampling rates of the organic micropollutants were positively correlated to their hydrophobicity. In this study, we paid particular attention to the determination of the lag time for each compound, which enabled to assess the minimum duration for a concentration peak of pesticides in surface waters to be efficiently integrated by the stir bars. In addition, desorption kinetics of two deuterium labelled pesticides were monitored under laboratory conditions. The corresponding elimination rate constants were calculated, and the isotropic exchange on the stir bar PDMS phase was demonstrated for both chemicals. These results allowed the authors to consider both compounds as PRC and the PDMS coated stir bars as tools for the determination of time-weighted average pesticide concentrations in surface waters.