The effects of a 1230-G static magnetic field on spontaneous discharge frequency and discharge pattern of principle cells in the cat's lateral geniculate body (LGB) were examined. In 45% of cells studied, a decrease in frequency was seen after the field was turned on. This progressed, even after the field was turned off, with return to baseline after an average duration of 250 s. Onset typically was 75 s after the field was activated, with maximum effect occurring 135 sec thereafter. In 67% of those cells which exhibited a decrease in frequency and in 50% of those which did not, a change in discharge pattern, as reflected by the interspike interval histogram, was seen. When present, this was manifested as a decrease in short interspike intervals. The change in the interspike interval histogram usually persisted longer than the change in frequency. The gradual onset and prolonged time course of changes in LGB cell activity suggest either an alteration in the synaptic ionic environment or in neurotransmitter availability. It is hypothesized that strong magnetic fields produce a partial realignment of diamagnetically anisotropic molecules within the cell membrane, thereby distorting ion-specific channels sufficiently to alter their function.