Abstract: A method of stabilizing the static magnetic field in a magnetic resonance imaging (MRI) system applies a radio frequency pulse to a test sample within the static magnetic field and an NMR signal is acquired from the test sample. A characteristic of a signal indicative of the NMR signal, such as the area, phase or the sum of the differences of sampled values is ascertained. In addition, the number of recurring features, such as zero-crossings, of the signal indicative of the NMR signal can be determined and a correction signal is generated to adjust the static magnetic field via a set of correction coils driven by a signal derived from the number of recurring features and the value of the characteristic. Apparatus for stabilizing the static magnetic field is also disclosed.