A new method for reversible cooling of the inferior olivary nucleus has been used in chronically prepared monkeys. Local olivary cooling depressed discharge of complex spikes of Purkinje cells in contralateral cerebellar cortex. Selective cooling of the principal olive (lateral and dorsal lamellae) produced movement oscillations at about 3-5 Hz of the contralateral arm during cooling in a monkey trained to make prescribed arm movements in the horizontal plane. The effects resemble those of dentate dysfunction. Selective cooling of the dorsal accessory olive and/or the overlying reticular formation, in 3 monkeys, produced during cooling a tendency for postural drift of the contralateral arm and for reduction of its movement amplitudes. These changes tended to vary together according to the degree of cooling. Arm oscillations did not occur. It is concluded that climbing fiber projections from the principal olivary nucleus are essential in the primate for optimal neocerebellar control of arm movements.