1. We examined the relationship of cerebellar Purkinje cell discharge to the scaling of kinematics during a voluntary motor learning paradigm. The study focused on whether the occurrence of complex spike (CS) discharge was associated with kinematic changes. Two primates (Macaca mulatta) were trained to move a cursor using a two-joint manipulandum over a horizontal video screen from a start target to one of four target boxes. The relationship between the cursor and the hand (gain) was changed, requiring scaling of movement distance to complete the task. As previously described, when the novel gain was presented over 100-200 movement trials the animals adapted their movements by using a strategy of scaling the amplitude and velocity of the first phase of the movement while keeping time to peak velocity constant. 2. The paradigm consisted of four different phases. A control phase at a gain of 1.0 was initially performed. The learning phase over the next 180-210 movements used one of four gains (0.6, 0.75, 1.5, or 2.0). Last, a testing phase involved 80% of 100 trials at the learned gain and 20% of the trials at the control gain of 1.0. The distance control phase consisted of using a gain of 1.0 but having the animal move to targets placed at the distance and direction the hand moved in the adapted state. 3. Simple spikes (SSs) and CSs of 141 Purkinje cells recorded primarily in the intermediate and lateral regions of zones V and VI in three cerebellar hemispheres from the two primates were recorded during the distance control, control, learning, and testing phases. Some cells were recorded in lobule VII and Crus I. CS activity increased during the learning phase, as documented previously. The increase in CS discharge occurred before or during the first 200-300 ms of the movement. This is the same time period in which the kinematic changes necessary for adaptation to the novel gain occur. Of 141 Purkinje cells recorded during the learning paradigm, 104 (74%) demonstrated significant increases in CS firing rate during the learning-testing phase. Of these 104 cells, 82 had statistically significant SS modulation. 4. Movement trials with CSs were separated from the trials without CSs. Aligning the kinematic and spike train data on movement onset, the average velocity profiles were subtracted from each other and a strict statistical criterion applied to test for the significance of any differences. Movement trials randomly sorted into two groups served as a control.(ABSTRACT TRUNCATED AT 400 WORDS)