Monkeys were trained to grasp a handle and move it in a horizontal arc to a central position by flexing or extending the wrist. A torque motor applied forces to the handle that switched at random intervals to alternately load flexor and extensor muscles. At each load switch, the handle was displaced transiently from the central position, and then moved back by the monkeys and held there steadily again. Recordings were made from cerebellar Purkinje cells (P-cells) whose simple spike (SS) activity was related to the task. The magnitude of one of the oppositely directed loads was then altered and the monkeys took about 12-100 trials with the novel load before performing as regularly as previously. During this period with one known and one novel load, some P-cells underwent increases in complex spike (CS) frequency at specific times after the load switch. This increased CS frequency would last for a similar number of trials as that taken by the monkey to adapt to the novel load before decreasing to near its previous level. Associated with the increased CS frequency ther were decreases in SS frequency that persisted after the CS frequency had decreased to near its previous level. These results are consistent with theoretical proposals that motor learning takes place in the cerebellum through changes in the strength of transmission of parallel fiber synapses on P-cells caused by the climbing fiber input. These results further suggest that climbing fiber firing causes a decrease in the strength of parallel fiber synapses.