STATEMENT OF PROBLEM: The main factor that determines the prognosis of restored pulpless teeth is preservation of sound dentin. PURPOSE: This study evaluated the residual dentinal thickness (RDT) of mandibular premolars after preparation of post space with Gates Glidden and ParaPost drills. MATERIAL AND METHODS: Twelve extracted single canal mandibular premolars were embedded in clear polyester resin to the cementoenamel junction (CEJ) in a muffle device. Three horizontal sections were made 1, 3, and 5 mm apical to the CEJ. Mesiodistal (MD) and faciolingual (FL) axes were carefully marked and the RDT was measured for each slice. Each tooth slice was reassembled in the muffle device with orientation pins, then secured with stabilizing pins. This procedure was repeated after enlarging the root canal to K-40 file and preparing the coronal root canal space with Gates Glidden drills and ParaPost drills Nos. 3, 4, and 5. RESULTS: Residual dentinal thickness in a MD direction was 3.77 +/- 0.51 mm in the unprepared upper slice and 2.23 +/- 0.31 mm in the No. 5 ParaPost drill prepared lower slice, for a difference of 41%. The corresponding values for the FL direction were 4.35 +/- 0.51 mm and 4.08 +/- 0.46 mm, respectively (6%). CONCLUSION: The average dentinal thickness 5 mm below the CEJ in the mesial and distal directions after post space preparation approached the accepted minimal 1 mm. A conservative approach to post space preparation was advocated.