The purpose of this study was to elucidate the adaptation in masticatory function by changing the cusp angle of the natural dentition of one person using a metal splint. During each experiment mandibular movements and EMG were recorded during a period of weeks. While chewing raisins as test food, mandibular movements as measured by the lower incisal point displacement were detected three-dimensionally using the Selspot system, and electrical activity of the masticatory muscles was picked up for the anterior temporal and masseter muscles, at the right and left using surface electrodes. The time schedule was composed of three terms: (1) before insertion of the splint, (2) for 20 days after insertion of the splint and then for 30 days after removal (Experiment-1), (3) for 14 days after reinsertion of the splint and then for 7 day after removal (Experiment-2). The measurements were done for the following 7 indices: (1) opening phase, (2) closing phase, (3) cycle time, (4) stopping phase (St), which is the period when the mandible was stationary during occlusal contact, (5) duration of EMG activity, (6) duration of EMG silence, (7) the period of EMG silence (IA), which is the period from the end of EMG activity to the end of the stopping phase. The following results were obtained 1) On changing the cusp angle, there was an increase in cycle time, closing phase, opening phase, duration of EMG activity, and duration of EMG silence, but the stopping phase was less than before insertion of the splint. These 6 indices showed a great variation and became irregular at first. 2) The irregularity of the masticatory function disappeared gradually, and then all indices approached the value before insertion of the splint from day 5 or day 20 at the latest. 3) Scarcely any reaction was detected in IA or in the ratio of cycle time and opening phase throughout the measurement period. 4) The indices of the reinsertion period (Experiment 2) approached the value before insertion more rapidly than the period of the insertion (Experiment-1). And the indices after removal quickly approached pre-insertion values. In short, the indices of the time factor of the masticatory function showed that the effect of changing cusp angle was handled as only disturbances. And once the first adaptation was completed, masticatory function responded more quickly to the same stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)