The spread of an impulsive discharge into the region contained between short coaxial electrodes has been investigated using currents of 20 kA peak value, and approximately 20 μsec period. Nitrogen and air were used at initial pressures between 0.1 and 4 torr.The discharge spreads azimuthally from a narrow arc channel formed at the breakdown point. Time resolved measurements of refractivity of the ionized and neutral gas were made by Schlieren and interferometer techniques. Streak and frame photographs gave information on the spread of visible light, and 8 mm microwaves indicated the position of regions of dense ionization. These gave values of the velocities of shock fronts and ionized regions consistent with the plasma spreading behind non-ionizing shock waves at velocities up to 6 × 10⁵ cm sec^-1, the plasma finally filling the whole inter-electrode space except for a sharply defined dark region where the shocks collided. This dark region is shown to be composed of a thin slab of cold dense gas.Energy losses due to radiation, conduction and electrode effects are calculated to be less than 30% of the total energy input 30 μsec after breakdown.A theory by Drabkina has been modified to apply to a simple two-dimensional uniform shock model. Despite the circular geometry employed in these experiments, the theory provides good predictions of the position and shapes of the shock fronts as a function of time.