The 3-D tensile and compressive forces exerted on a tooth were measured in vivo during function using a force-measuring device including a piezoelectric transducer. The device was mounted on the maxillary left second molar of a healthy male subject; the subject tooth had been endodontically treated and prepared for metal abutment and a crown. The 3-D forces were expressed as a vector of the coordinates based on the Frankfort horizontal (x-y) and sagittal (y-z) planes. The device captured the sequential changes in the forces. The directions of the forces changed during not only chewing a caramel or a peanut but also maximum voluntary clenching (MVC). As the magnitudes of the force increased during both MVC and caramel chewing (CaC), the force vector tended to correspond to the direction of the palatal root, medially and posteriorly. The compressive forces during MVC and caramel and peanut chewing were 173.29+/-15.32, 146.3+/-14.7 and 57.7+/-35.7 N, respectively. The force vector during MVC was directed from the crown to the root medially at an angle of 10.27+/-1.00 degrees from the y-z plane and posteriorly at an angle of 3.18+/-0.85 degrees from the x-z plane to the perpendicular line of the F-H plane. There were significant differences in the behaviour of the compressive forces between clenching and chewing. The tensile force was recorded during CaC, not peanut chewing.