The accuracy of saccades is ensured by an adaptive mechanism that probably involves the cerebellum. We examined the discharge of saccade-related neurons in the fastigial oculomotor region (FOR) during adaptation. Using a conventional intrasaccadic step paradigm, we changed the gain of saccades elicited by a 10 degrees horizontal target step to the side of unit recording. As a measure of neural activity, we took the number of spikes occurring in a 30 or 40 ms time window starting at 30 ms before saccade onset, which corresponded roughly to the foot and rising phase of the burst. A gain decrease was accompanied by a significant increase in spike discharge (6/6), and a gain increase by a significant reduction in discharge (3/3). During the course of adaptation, the neural activity and gain exhibited changes with a similar course but in the opposite direction. Regression analysis indicated that the two variables were significantly correlated (7/8). The present study has shown that activity of FOR neurons is altered during adaptive modification of saccade size. Our data are consistent with the hypothesized suppressive action of the FOR on ipsiversive saccades and provide support on a single-neuron basis for the cerebellar involvement in short-term saccade adaptation.