||The database for medium- and long-term model validation using Sb-125 released by the La Hague reprocessing plant includes 1400 measurements performed between 1987 and 1994 in the English Channel and the North Sea and data for each release since 1982. Antimony-125 has a conservative behaviour in water masses over a period of several years. These data can be used qualitatively and quantitatively to compare the measured concentrations with the calculated ones and quantities of tracers. Tritium measurements are also available for model calibration. A two-dimensional hydrodynamic model has been developed to allow repetitive long-term simulations. This model uses a database of residual tidal currents calculated using the Lagrangian barycentric method [Salomon, J.C., Guegueniat, P., Orbi, A., Baron, Y., 1988. A Lagrangian model for long-term tidally induced transport and mixing. Verification by artificial radionuclide concentrations. In: Guary, J.C., Guegueniat, P., Pentreath, R.J. (Eds.), Radionuclides: A Tool for Oceanography, Cherbourg 1-5 June, 1987. Elsevier Applied Science Publishers, London, New York, pp. 384-394]. The area covered by the model includes the English Channel, the southern North Sea and the Irish Sea with a mesh size of 1 km. The main adjustment parameters of this model are the sources of wind data used and the calculation method for evaluating wind stress at the sea surface. With these parameters, the fluxes of radionuclides and water masses in the English Channel and the North Sea were balanced for the whole period of field measurements (1987-1994). The correlation factor between individual measurements in seawater and calculation results is 0.88 with an average error of +/-54%, the error attributable to the measurement process being 15% on average. The mean flux through the Dover Strait is 126,000 m(3) s(-1), close from the one obtained from previous studies [Salomon, J.C., Breton, M., Guegueniat, P. 1993. Computed residual flow through the Dover Strait. Oceanologica Acta 16 (5-6) 449-455; Bailly du Bois, P., Salomon, J.C., Gandon, R., Guegueniat, P. 1995. A quantitative estimate of English Channel water fluxes into the North Sea from 1987 to 1992 based on radiotracer distribution. Journal of Marine Systems 6 (5-6) 457-481; Prandle, D., Ballard, G., Flatt, D., Harrison, A.J., Jones, S.E. Knight, P.J., Loch, S. McManus, J. Player, R. Tappin, A., 1996. Combining modelling and monitoring to determine fluxes of water, dissolved and particulate metals through the Dover Strait. Continental Shelf Research 16 (2) 237-257]. This model appears as a robust tool for modelling long-term dispersion at a low computation cost (it takes less than I h to simulate one year on a personal computer). It could forecast consequences of controlled or accidental releases of soluble substances in seawater under realistic conditions or calculate water masses fluxes and transit times.