Vestibuloocular reflex (VOR) was induced by horizontal sinusoidal whole-body rotation in darkness in rabbits and a monkey. One eye was observed through an infrared TV camera. The gain of VOR was adaptively changed when the animal was continuously rotated for 3 h with the observed eye exposed to the screen moving in phase or out of phase with the head. Injection of 0.1 ml saline solution containing 10 microM hemoglobin into the subdural space over the cerebellar flocculus ipsilateral to the observed eye abolished the VOR adaptation. Since hemoglobin absorbs nitric oxide, which mediates synaptic plasticity in the cerebellar cortex, these results support the view that synaptic plasticity of the flocculus plays a key role in the VOR adaptation.