The learning machine model of the cerebellum by Marr and Albus contains a special type of synaptic plasticity. Experimental evidence for this synaptic plasticity has been meager, but very recently positive evidence has become available. Ito, Sakurai and Tongroach (7) demonstrated the occurrence of a long-lasting depression in mossy fiber responsiveness of Purkinje cells subsequent to conjuctive stimulation of mossy fibers and climbing fibers. A similar long-lasting depression was shown to occurred in sensitivity of Purkinje cell dendrites to a putative neurotransmitter of parallel fibers, i.e., glutamate. Furthermore, Ito and Kano (6) produced a long-lasting depression in the molecular layer of the cerebellar cortex by simultaneous direct stimulation of parallel fibers and climbing fibers. These long-lasting depressions appear to represent a synaptic plasticity of the form proposed by Albus.