The results from several investigations suggest that climbing fibres heterosynaptically depress parallel fibre responses in Purkinje cells. In the present investigation the mechanism behind the depression has been studied by extracellular recording of responses in single Purkinje cells, evoked by electrical stimulation of parallel fibres and climbing fibres. The results show that a short time of conjunctive stimulation of climbing fibres and parallel fibres results in a long-lasting depression of the parallel fibre responses and that this depression can be prevented if the Purkinje cells are inhibited during the conjunctive stimulation. Since inhibition has been shown to shorten or abolish the long-lasting plateau potentials which are evoked by climbing fibre impulses this finding supports the assumption that the climbing fibre evoked plateau potentials mediate the heterosynaptic depression of parallel fibre responses.