We used visuomotor tracking as our motor task and studied how subjects learn to adjust for inversion of the relation between joystick movement and target movement. This task requires learning a novel sensorimotor transformation. We have measured tracking performance and pupil dilation simultaneously. We have used pupil dilation as a measure of cognitive load, since the diameter of the human pupil increases with task difficulty across a wide range of cognitive tasks. Subjects observed a target moving at constant velocity along a clockwise circular trajectory on a computer screen. Subjects held a joystick in their hand, and moved it so that a cursor tracked the target as closely as possible. 60 normal subjects participated in the experiment. During 6 blocks of learning, inversion-evoked tracking error and inversion-evoked pupil dilation both decreased significantly. This finding suggests increasing automatization of the to-be-learned sensorimotor transformation. Pupil measures were not correlated with tracking error on individual trials, suggesting that the inversion-evoked cognitive load reflects changes in motor task, and is not merely a response to high errors. Our results thus suggest a relatively direct physiological measure of the processes of motor-skill automatization. © 2009 Springer-Verlag.