30% of all children show a class II occlusion. This occlusion anomaly is normally treated with activator. The remodeling operations in the muscles of mastication are still insufficiently solved. Pigs show a chewing pattern similar to human. 10-weeks old pigs were used for the present treatment. Animals were divided into 2 groups of 6 each. Treatment (with build-ups) was realized during a period of 4 weeks. Genetic changes in the musculature were registered by Northern blotting and the polymerase chain reaction (PCR). Two isoforms of the Myosin Heavy Chain of the different muscle fiber types I and II (slow twitch and fast twitch) were examined in order to demonstrate stress of muscles. Differences in mRNA- content were proved in all muscles of mastication. Treatment conditional adaptation of one fiber type into the other was realized by PCR. By this the adaptation of the muscles of mastication after sagittal advancement of the lower jaw was genetically proved in the animal experiment for the first time. Keywords: training effects, Myosin, muscle fiber types