1. Nitric oxide (NO) production in the nervous system has been implicated in cellular mechanisms of learning and memory. Our study investigates an in vivo sensorimotor model of learning. It demonstrates that a localized vestibulocerebellar injection of the NO synthase inhibitor, L-NG-monomethyl-arginine (L-NMMA), which specifically blocks NO production, inhibited the acquisition of adaptive vestibulo-ocular reflex (VOR) gain increases but not gain decreases in the goldfish. 2. Restoration of NO production by concomitant administration of L-arginine (the substrate for NO synthase) and L-NMMA suppressed the inhibitory effect of L-NMMA on adaptive gain increases. 3. This effect of L-NMMA was stereospecific because injection of D-NMMA did not suppress adaptive VOR gain increases. 4. Injection of L-NMMA after VOR adaptation had no effect on retention, failing to alter the postadaptive recovery after a VOR gain increase. 5. In conclusion, acquisition of adaptive VOR gain increases are affected by cerebellar NO inhibition. However, because gain decreases are not, they may involve either non-NO cerebellar or extracerebellar mechanisms. In addition, different processes for acquisition and retention of gain increases may be operating, because inhibition of cerebellar NO affects the acquisition but not the retention phase.