The activity of primary horizontal semicircular canal neurons in newborn rats was compared to similar cells in adults. All animals were anesthetized with ether. Neurons were categorized as regular or irregular on the basis of their spontaneous activity. Horizontal semicircular canal neurons can respond to angular acceleration stimulation at birth. In newborn rats no regularly firing cells could be found, but the percentage of these cells and their average resting rate increased during growth. Neurons in newborn rats differ from those in the adult by having a lower average resting rate, a lower sensitivity to long-duration angular acceleration and taking longer to reach peak increase in firing during the stimulus. Sensitivity reaches adult values by about 4 days although the canal dimensions continue to increase until about 20 days.