The interior of carbon nanotubes, provide a homogeneous, contaminant-free environment in which to perform nanoreactions, hence the term nano-test tubes. Here, routes for the controlled growth of carbon nanotubes with defined chiral indices are presented within the interior of single-walled carbon nanotubes upon the encapsulation of Pt and Fe catalysts, plus the carbon source in controlled proportions. Factors affecting the growth (e.g. temperature) are examined experimentally.