Abstract: Variations in the Brunt–Väisälä frequency caused by mesoscale eddies of different polarity are analyzed. The Lofoten Basin in the Norwegian Sea is the study region. Two eddies are considered: the Lofoten eddy, an anticyclone located in the center of the basin, and a cyclone located southeast of the Lofoten eddy. The data of the GLORYS12V1 oceanic reanalysis for June 10, 2010, are used in the analysis. In the considered eddies, profiles, vertical sections of potential vorticity, and the Brunt–Väisälä frequency, as well as the associated characteristics of seawater, are investigated. Theoretical estimates of the Brunt–Väisälä frequency restructured by eddies of different polarity are proposed. It is shown that potential vorticity of anticyclones should have a limit. The properties of the considered eddies, which cannot be measured directly and cannot be found from the models, are estimated indirectly by joint analysis of field measurements and theoretical material. The article is divided into two parts. The first part presents the elements of the theory and derives and discusses the mathematical relations used in the second part needed to calculate the characteristics of eddies of the Lofoten Basin from field data.