Single-walled carbon nanotubes (SWNTs), prepared by metal-catalysed laser ablation of graphite, form close-packed bundles or ’ropes’(1). These rope crystallites exhibit metallic behaviour above 50 K (ref. 2), and individual tubes behave as molecular wires, exhibiting quantum effects at low temperatures(3,4). They offer an ah-carbon host lattice that, by analogy with graphite(5) and solid C-60 (ref. 6), might form intercalation compounds with interesting electronic properties, such as enhanced electrical conductivity and superconductivity. Multi-walled nanotube materials have been doped with alkali metals(7) and FeCl3 (ref. 8). Here we report the doping of bulk samples of SWNTs by vapour-phase reactions with bromine and potassium-a prototypical electron acceptor and donor respectively Doping decreases the resistivity at 300 K by up to a factor of 30, and enlarges the region where the temperature coefficient of resistance is positive (the signature of metallic behaviour). These results suggest that doped SWNTs represent a new family of synthetic metals.