Homing pigeons appear to use the earth's magnetic field as a compass and perhaps as part of their position-finding system or 'map'. The sensory system they use to detect magnetic fields is unknown, but two current possibilities are some mode of response by the pineal organ or by the visual system, or it may be based on the magnetite crystals found in their heads. Three series of experiments to test the involvement of magnetite are reported here. The alignment of the permanent magnetic domains in the birds heads was altered by (a) demagnetizing the birds, (b) magnetizing them with a strong magnetic field and (c) exposing the birds to a strong magnetic gradient. None of these treatments had a marked effect on the pigeon's orientation or homing under sunny skies, but a few results obtained under overcast skies suggest that demagnetizing the birds may have increased the scatter of their vanishing bearings. Perhaps pigeons use one magnetic sensor for their magnetic compass and another for some component of the map.