This study evaluated the apical movement of gutta-percha master cones under in vitro vertical compaction. A split natural tooth model was used. The gutta-percha cones were first fitted at 0.5 and 1.0 mm short of the apical end of a canal instrumented up to a size 25 file and then condensed. Then the canal was cleaned of gutta-percha and reshaped up to a size 40 file, other cones fitted and obturated. Photographs were taken before and after compaction and measurements were made. The canals instrumented to a size 40 file had significantly more overextensions than those instrumented to a size 25 file.