The invention relates to a method and an apparatus for influencing magnetic particles in a region of action. With the help of an arrangement that has means for generating magnetic fields, a spatially inhomogeneous magnetic field is generated that has at least one zone (301) in which the magnetization of the particles is in a state of non-saturation, whereas it is in a state of saturation in the remaining zone. By displacing the said zone within the region of action, a change in magnetization is produced that can be detected from outside and that gives information on the spatial distribution of the magnetic particles in the region of action. Alternatively, the displacement may also be repeated so frequently that the region of action heats up. To improve the accessibility of the region of action, the said region is situated outside the arrangement having means for generating magnetic fields.