In rabbit cerebellar cortex, local stimulation of parallel fibers induced field potentials with two negative peaks, n1 representing conducting spikes of parallel fibers and n2 postsynaptic excitation in dendrites of Purkinje cells and other cortical cells. Conjunctive stimulation of parallel fibers and climbing fibers at 4 Hz for 30-120 sec caused a significant depression of n2 potential which lasted for at least 1 h. Such an effect could not be produced by stimulation of climbing fibers or parallel fibers alone. These observations support the plasticity assumption in the Marr-Albus model of the cerebellum.