In cerebellar Purkinje cells, conjunctive stimulation of parallel fibers and the climbing fiber causes long-term depression of parallel fiber-Purkinje cell transmission. It has been postulated that calcium is an intracellular mediator of the climbing fiber to induce this synaptic modification. To directly test the hypothesis, a calcium-chelating agent, EGTA, was intracellularly injected into Purkinje cells. In these injected cells, conjunctive stimulation failed to induce depression. Instead, it caused potentiation similar to that observed after repetitive stimulation of parallel fibers alone.