The paper deals with the first works for the application of the Hole-Drilling technique on timber structures in service. The final objective of the research is the deduction of the stresses in structural elements of Architectural Heritage. The method involves glued strain gages at the surface of a structural element. At least three strain gages are placed in the area under consideration in order to record the strains originated by stress relaxation after eliminating a round of 18 mm diameter and 16 mm depth. The stresses before drill can be deduced with an appropriate mathematical scheme. The Hole Drilling technique can be considered as minor destructive test providing sufficient information to calculate the principal stresses and their directions around the drill. At laboratory four wood specimens (blocks) have been loaded under known stresses in order to compare them with those stresses obtained by the hole drilling procedure. The results show some desviation because the mathematical scheme assumes the blocks as isotropic material. In this sense, the research described has to be improved to take into account the orthotropyc behavior of the wood as structural element.