Equilibrium passive sampling through in vivo implantation can help circumvent complex extractions of biological tissues, provide more accurate information on chemical contaminant burden based on the fugacity of a chemical in an organism rather than conventional normalization to lipid content, and improve the assessment of contaminant bioaccumulation potential. Here, we explored the feasibility of in vivo implantation for the passive sampling of neutral hydrophobic contaminants through the insertion of a silicone tag into brown trout (Salmo trutta). Implanted fish from the upper reaches of the River Alna (Oslo, Norway) were relocated to a polluted section of the river for a 28 day caged exposure. "Whole fish" lipid-silicone distribution coefficients (Dlip-sil) were calculated for chlorinated compounds measured in whole fish and in silicone tags of 13 fish. Dlip-sil ranged from 13.6 to 40.0 g g(-1) for polychlorinated biphenyl congeners 28-156 (CB28 and CB156), respectively, and are in close agreement with literature in vitro lipid phase and tissue-based lipid-silicone partition coefficients. After dissection a further of eight fish, muscle and liver samples were analyzed separately. Muscle-based Dlip-sil values similar to the whole fish data were observed. However, lipid-normalized concentrations in the liver tended to be lower than in muscle for most compounds (by up to 50%). Values of whole fish Dlip-sil for brominated diphenyl ethers determined for three fish were in the range of 8.6-51 g g(-1) and in agreement with chlorinated substances. Finally, fugacity ratios calculated from equilibrium concentrations in fish-implanted and water-exposed silicone provided information on the bioaccumulation for chlorinated compounds as well as for some polycyclic aromatic hydrocarbons. Equilibrium passive sampling through in vivo implantation can allow the comparison of a chemical's activity or fugacity in biotic as well as abiotic environmental compartments and at different trophic levels up to humans.