abstract–An attempt to study the eruption of the guinea pig molars through the oral mucosa resulted in the observation that the guinea pig is born with a functioning and heavily attrited dentition. Extensive occlusal wear of the molar teeth was likewise found in guinea pig fetuses about ten days prior to birth. These findings indicate that clenching and grinding of the teeth may occur without repressed aggressions or emotional tension and that dietary abrasives are not necessary for the attrition of the occlusal tooth surfaces. The present observations thus support the view that occlusal wear is not only a mechanical loss of tooth substance but, in most animals, an indispensable biologic occurrence in the development and maintenance of a functional masticatory orga