We used positron emission tomography (PET) to compare the contribution of the cerebellum and basal ganglia to the sensory guidance of movement. In one condition the subjects used a computer mouse to draw a series of lines on a computer screen (DRAW). In the second condition the same lines were presented to the subjects, and they had to track the lines with a mouse pointer on the screen (COPY). In a third condition the subjects were again presented with the same lines, and they simply followed movements of the pointer with their eyes (EYES). In the fourth condition, the subjects fixated a central point, ignoring the sequence of presented lines (FIX). The pons and cerebellum were activated more during visually guided tracking than in freely generated drawing (COPY vs DRAW). The basal ganglia were activated equally in both DRAW and COPY. The prefrontal and inferior temporal cortex were activated more when subjects drew lines freely (DRAW) than when they copied them (COPY). We conclude that the cerebellum is specialized for using sensory information to correct movements, but that the basal ganglia are involved both in movements that are self-generated and in movements that are guided by external cues.