The larvikite complex southwest of the lake Gjerdingen has been studied on the basis of its magnetic properties. The mean declination and inclination of the remanent magnetization are 199° and -45°, respectively. A negative magnetic anomaly associated with the larvikite is due to a dominant remanent magnetization within the medium grained larvikite which contains a relatively high concentration of magnetite. The influence of topography on magnetic measurements is discussed. The magnetic model gives a near vertical extension of surface features to a depth of approximately 1 km where the magnetization contrast vanishes. This implies that the larvikite is either an isolated plug floating in the nordmarkite or the complex widens below this depth.