Recent work in our laboratory has demonstrated that spontaneous or evoked climbing fiber inputs are associated with an increased responsiveness of Purkinje cells to mossy fiber inputs activated by natural peripheral stimuli. These experiments were designed to test the hypothesis that this increased responsiveness occurs in the cerebellar cortex. In decerebrate, unanesthetized cats Purkinje cells in the surface folium were isolated and their simple and complex spikes discriminated. A bipolar concentric stimulating electrode was placed on the surface of the folium to activate the parallel fiber volley and modulate the Purkinje cell's simple spike discharge. The cell's simple spike discharge was evaluated when the surface stimulus was presented randomly and when this stimulus was timed to occur at a fixed interval after the spontaneous complex spike activity. When the surface stimulus was timed to occur at short intervals after the spontaneous complex spike, the response to the surface stimulus was accentuated. This increase in simple spike modulation occurred independent of whether the simple spike discharge increased or decreased in response to the surface stimulus. These results support the hypothesis that the climbing fiber input changes the gain of Purkinje cells' simple spike responses to mossy fiber inputs due to interactions occurring in the cerebellar cortex.