The start-up of a large CTR tokamak plasma at low density using energetic neutral beams is studied. Questions of beam-energy deposition profiles, beam-energy requirements for penetration, and beam-power requirements to achieve ignition or prescribed plasma heat-up rates are examined. A detailed analysis for a particular CTR-system design, using a two-fluid, space-time numerical model to simulate plasma behaviour, is reported. The numerical model includes neoclassical ion conduction, pseudoclassical electron conduction, radiation losses, Ohmic heating, thermonuclear alphaparticle heating, and injection heating. It is found that even when the power density deposition profile for beam heating is peaked on the axis, the plasma temperature profiles can have local maxima and thus be inverted in the outer plasma regions. This is caused by the decreased density in these zones. For a specific large system studied in detail (aspect ratio 2.6, plasma radius 500 cm, initial peak density 3 × l013/cm3), beam energies of 100 keV, 350 keV, and 500 keV are considered. Only the 500-keV beam case yields non-inverted temperature profiles during heating. In all cases, moderate power levels are required to ignite low-density plasmas in times of the order of seconds. In addition, the injection time required to establish a prescribed plasma heat-up rate is examined in detail. In some cases, for a given amount of injected energy, the heat-up rate resulting from low-energy beams is greater than that due to higher-energy beams. Beam-energy and -power requirements for smaller systems are also given.