A three-dimensional, baroclinic, hydrodynamic model was based on the coastal ocean circulation model (Blumberg and Mellor 1987) known as POM (Princeton Ocean Model), which was adapted to Baltic conditions. The way of horizontal advection calculation has been modified by using TVD filtration (Total Variation Diminishing) thus oscillations in frontal zones have been elimi-nated. To predict water exchange between the Gulf of Gdansk and the open sea, the model covers the whole Baltic Sea together with the Danish Straits. The model has been verified based on three-year simulation of spreading of freshwater introduced by 49 biggest rivers. The influence of wind has been taken into account assuming the variable fields of shear stress every 6 hours. Thermic conditions have been affected by heat fluxes calculated from meteorological data. Calculated and measured salinity and temperature distributions are in relatively good accordance for the surface layer. Seasonal changes in vertical distributions of temperature as well as formation and disappearance of thermocline have been approximated properly.