The present study demonstrates that operant behavior is affected by a combination of a 60-Hz magnetic field and a magnetostatic field 2.6 X 10-5T (about half the geomagnetic field). Rats exposed to this combination for 30 min consistently exhibited changes in the rate and pattern of responding during the differential reinforcement of low rate (DRL) component of a multiple fixed ratio (FR) DRL reinforcement schedule. By contrast, there were no measurable changes following exposure to the static field alone or to the oscillating field alone, even with a 10-fold increase in intensity (5 X W 5 to 5 x Tms). A cyclotron resonance mechanism has been suggested as a possible explanation for the observation that weak static magnetic fields modify the response of in vitro brain tissue to low-frequency magnetic fields. The choice of static field intensity B, and frequency v in the present study follows from the cyclotron resonance condition Y = (1/2?r)(q/m)B,, for singly charged lithium, an element in extensive use in the clinical treatment of affective disorders in humans. The present research is consistent with a cellular cyclotron resonance mechanism and tends to imply a functional dependence of behavior on the geomagnetic field.