The shallowness of the ¿resund prevents a continuous inflow of saline Kattegat water to the Baltic. Instead, the salt is exchanged largely by fluctuating barotropic transports. Buffer effects and temporal storage of low-salinity Baltic surface water complicate the exchange. The analysis of salt exchange through the ¿resund requires use of a model of the stratification and baroclinically modified exchange processes. In this paper a three-layer model of the ¿resund, forced by the exchange with the Kattegat and the Baltic, is formulated and calibrated. Frontal dynamics, necessary to explain the retreat of the uppermost layer, are included. The model is calibrated using genetic algorithms, which provide an efficient and robust optimization method for this kind of model. An analysis of the exchange in view of the model results is presented. The paper also gives estimates of typical mean quantities. For the analyzed period of 11 years (1977--1987) the mean salt outflow rate from the Baltic during outflows is 311,000 kg s-1, and the mean salt inflow rate to the Baltic during inflows is 500,000 kg s-1. The net salt outflow from the Baltic is estimated to 19,000 kg s-1. The mean frontal speed is estimated at 0.25 m s-1 and the typical required length of an inflow event for high-salinity Kattegat water to reach the Baltic is estimated at 4 days. Further results are also given. In addition, the baroclinic dynamics of the ¿resund are discussed. ¿ American Geophysical Union 1996