Extracellular recordings using glass microelectrodes were made from the ophthalmic and supraorbital nerves of a transequatorial migratory bird, the bobolink. The rate of electrical activity was modified in 15% of the spontaneously active units by earth-strength alterations of the horizontal or vertical component of the ambient magnetic field using box coils. Responses to magnetic stimulation included augmentation or inhibition of spontaneous activity, or an ON-OFF or OFF response. Responses to magnetic stimulation were also recorded from the optic tectum and the pineal gland. The responses of the trigeminal system are probably independent of the visual system and indicate the presence of two separate magnetic receptor systems in one avian species. The responses from the trigeminal receptor may involve magnetite for transduction of magnetic field information.