Objectives: The aim of this research was to study the longitudinal relation between mesiobuccal and mesiolingual canal ends of the mandibular first molar mesial root, beyond details its internal anatomy. Methods: Eighty mesial roots of mandibular first molars extracted of adult humans were diaphanizated by technique using 10% formaldehyde for five days; 5% NaOCl for three days; India ink infiltration by blood's centrifugal; stove at 60° C; decalcification in 5% HCl; dehydration in increasing concentrations of ethyl alcohol; immersion in xylene solution; immersion in methyl salicylate. The examination of the diaphanizated roots happened through microscope stereoscopic, using ocular micrometer for longitudinal mensuration between foraminal opens of the MB and ML canals. Results:The results showed foraminal ends with following differences: On the same level (38.75%); 0.1mm to 0.5mm (35.00%); 0.6mm to 1.0mm (12.50%); 1.1mm to 1.5mm (8.75%); 1.6mm to 2.0mm (3.75%); 2.1mm to 2.6mm (1.25%). In the gross of the sample was also detected the presence of: Intercanals (43.75%); recurrents (10.00%); secondaries (13.75%); laterals (1.25%); apical deltas (12.50%); simple canals (2.50%); two canals with a simple forame (16.25%); colaterals and 3° canal (6.25%); eccentric forames (3.75%); isthmus infiltrated until the apical third (2.50%). Conclusions: We conclude that the new details here preesented, as isthmus, MV-ML relation, and 3° canal, they will have contribute too much to improve the endodontics teaching, showing at the students important increment to the literature statistics data. (Financial support: FAPEMIG).