We examined pharmacologically whether cerebellar long-term depression (LTD) may play a role in the adaptation of smooth pursuit eye movements in two Macaca fuscata, which were trained to pursue a target moving in the horizontal plane in a 3 degrees step-10 deg/s ramp mode. The monkeys showed small catch-up saccades followed by 6-8 deg/s post-saccadic pursuit movements. Adaptation of the post-saccadic pursuit velocity was induced by repetition of acceleration of the target to 20 deg/s after the catch-up saccades. Injections of 0.1 mM hemoglobin or 20 mM NG-monomethyl-L-arginine solution into the subdural space above the paraflocculus-flocculus scarcely affected the post-saccadic pursuit velocity, but markedly depressed its adaptation. These observations suggest that cerebellar LTD may underlie the adaptation of smooth pursuit.