The present invention is an apparatus for use in drilling operations. It uses an under-reamer having a plurality of elongated arms with cutting elements at the ends of the arms for enlarging a previously drilled borehole drilled by a drill bit. One or more stabilizers in close proximity to the under-reamer provide stability to the under-reamer and the drill bit. The stabilizer could be rotating or non-rotating; and could be positioned between the under-reamer and the drill bit, or above the under reamer or above a directional device on the drillstring. The cutting arms are selectively operable to perform the enlargement. The stabilizer may be provided with members that closely fit the size of the borehole.