The results of experimental and numerical studies of the generation of runaway electrons (RAE) in a pressurized air-filled diode under the application of 20ns-, 5ns- and 1ns- duration high-voltage pulses with an amplitude up to 160kV are presented. It is shown that with a 1 ns pulse, RAE with energy ³20keV reach the anode prior to the formation of the plasma channel between the cathode and anode. Conversely, with 20ns or 5ns pulses, RAE with energy ³20keV were obtained at the anode only after the formation of the plasma channel. In addition, the high- and low-impedance stages of the development of the discharge were found. Finally, a comparison between experimental and numerical simulation results is presented.