The neodymium:yttrium-aluminium-garnet (Nd:YAG) laser is currently used in dental laboratories to weld metals on dental prostheses. Recently, the use of Nd:YAG has been suggested so that dentists themselves can repair broken fixed, removable and orthodontic prostheses by welding metals directly in the mouth. This work aimed to evaluate, through a four k-type thermocouple system on calf jaws, the thermal increase in the biological structures close to the metal parts during laser welding. We put two hemispherical metal plates onto mandibular molars and then laser welded them at three points with a four k-thermocouple system to determine the thermal rise in the pulp chamber, sulcus, root and bone. This procedure was carried out on 12 samples, and the results were processed. The highest values of thermal increase were found in the pulp chamber, 1.5 degrees C; sulcus, 0.7 degrees C; root, 0.3 degrees C; and bone, 0.3 degrees C. This study showed that thermal increases in pulp chamber, sulcus, root and bone were biologically compatible and that intra-oral laser welding, at the parameters used in this work, seems to be harmless to the biological structures close to the welding and thermally affected zones.