Fast gas heating is studied experimentally and numerically using pulsed nanosecond capillary discharge in pure nitrogen and N 2 :O 2 mixtures under the conditions of high specific deposited energy (up to 1 eV/molecule) and high reduced electric fields (100-300 Td). Deposited energy, electric field and gas temperature are measured as functions of time. The radial distribution of active species is analyzed experimentally. The roles of processes involving = ( )u and N( 2 D) excited nitrogen species leading to heat release are analyzed using numerical modeling in the framework of 1D axial approximation.