Total magnetic fields were recorded at five sites on Mt. St. Helens between 23 October and 11 November 1981, an interval which included an extrusive dome-building eruption of the volcano. Two of the magneto-meters located in the volcano's crater measured reversible magnetic changes, which correspond to fluctuations in tilt measured nearby. However, the relationship is highly nonlinear. Electric fields were measured on the east flank of the volcano near its summit to search for electrokinetic effects. They show no correlation with the magnetic changes and, in the long term, are uncorrelated with eruptive activity. Our favored interpretation of the magnetic changes is that they result from stress-induced changes in the magnetization of the volcano. Magnetic field values returned to pre-anomaly values. This reversibility rules out pressure-induced magnetization as the dominant mechanism and places an upper limit of &sgr;~300 bars on the stress changes. The limited spatial extent of the magnetic anomaly field places the source of stress at shallow depth beneath the crater floor consistent with models based on strain data.