Carbon nanotubes have been grown on silicon substrates coated with nickel-catalyst films by chemical-vapor deposition using mixtures of acetylene and nitrogen. An effective aging treatment was carried out before the field-emission measurement of carbon-nanotube samples. The results show that the turn-on field is 1.8 V/mu m at 2 mu A emission current and that the highest emission current of 13 mA at the electrical field of 23 V/mu m. This corresponds to the field-emission current density of 414 mA/cm(2). Some discussions about the long growing time of carbon nanotubes, the aging process, and high-current field emission are given. (c) 2007 American Vacuum Society.