The conversion efficiency of laser energy into thermal energy of a gas as applied to laser rocket engines was discussed. The experimental results of the conversion efficiency of the energy pulses of gas and solid-state lasers into thermal energy of gas accompanied by breakdown in nitrogen and air at pressures of 0.1-1 atm and pulse energies of 5-90 J were presented. The results showed that the breakdown threshold of atmospheric-pressure nitrogen is (4-6) × 10⁹ W/cm² for a CO₂-laser pulse and is a factor of 2 greater than the breakdown threshold of air.