Field potentials and extracellular potassium concentration ([K+]o) were simultaneously monitored in the molecular layer of the rat cerebellar cortex during stimulation of the parallel fibers. The synaptic field potential elicited by stimulation was reduced by several methods. Reduction of synaptic field potentials was accompanied by a marked increase in the excitability of the parallel fibers. This change in excitability was related to the degree of extracellular K+ accumulation associated with parallel fiber stimulation. These findings support the proposal that increases in [K+]o associated with activity in postsynaptic elements can modulate the excitability of presynaptic afferent fibers.