In four subjects the electrical activity in the anterior and posterior temporal and masseter muscles during maximal bite was recorded bilaterally with and without premature unilateral contact. Muscle activity was measured as the average level and the peak of the mean voltage with layers of strips of 0.05, 0.10, 0.15 and 2.0 mm, placed between first molars either on the left or the right side, and compared with the level of activity with undistrubed occlusion. Unilateral premature contact caused a significant asymmetry of action in all muscles under study with stronger activity ipsilaterally. With increasing thickness of the overlay the mean voltage decreased in parallel on both sides. We suggest that the asymmetry was caused by larger spindle afferent activity on the ipsilateral as compared to the contralateral side, and that the all-over decrease of muscle activity was due to a gradual reduction of activity from periodontal pressoreceptors.