Total magnetic field data have been measured and interpreted along selected profiles in an area through the Jämtland-Trøndelag Caledonides. The object has been to model local and regional structures and investigate their possible relevance for interpretation of Caledonian tectonics. Two different methods of interpretation have been applied: analytic signal analysis, and fitting of polygonal models: they yield partly consistent, partly supplementary results. With few exceptions, the Caledonian cover units are weakly magnetized and are only responsible for very local anomalies. The main anomalies derive from the autochthonous-parautochthonous crystalline basement. The prominent regional anomaly in the area (the Jämtland Anomaly) is found to originate from the Proterozoic sequence of Dala granites and volcanics that crop out in the autochthon south of the area and obviously continue northwards as a basement below the Caledonian rocks. It forms a huge rock volume, 500 km long, 200 km wide and about 10 km thick. The assigned magnetic susceptibility, 0.075 SI-units, is supported by the existing rock magnetic data. The magnetic model of the basement surface below the Caledonian cover shows a pattern of synforms and antiforms in general agreement with the existing geological information. However, a previously unpredicted antiform is found at the eastern front of the Offerdal Nappe near Landön. Caledonian activity appears to have shaped the basement surface. It may also be responsible, at least partly, for some internal structure in the Precambrian basement, such as is seen in the alignment of the regular linear western flank of the Jämtland Anomaly with the western limb of the Sylarna Antiform.