Experiments were conducted to determine the variation of time lag of spark breakdown as a function of gas pressure. All tests were made in pre‐purified nitrogen using 1.250‐inch diameter spheres of copper, aluminum and nickel with a gap spacing of 0.0125 inch. The voltage applied to the gap was in the form of a surge which reached its maximum value in one quarter of a microsecond and which decayed slowly to 97 percent of its maximum value in 5000 microseconds. Most of the recorded lags lay in the range from 5 to 5000 microseconds. The maximum value of the applied voltage was varied from 14 to 128 percent above the static breakdown value, while the gas pressure was varied from 7 to 100 pounds per square inch absolute. The most probable time lag was found to vary greatly with pressure, changing rather abruptly from a large value (10−2sec.) to a small value (10−5−10−6sec.) at certain critical conditions of pressure and overvoltage. The change appears to occur under conditions at which the ions normally present in the gas begin to produce free electrons by negative‐ion disintegration, or by positive‐ion bombardment of the cathode. The distribution of time lags was found to vary with cathode material and condition of cathode surface. The nickel spheres quickly developed a heavy coating when sparked in nitrogen. The copper and aluminum spheres became only slightly discolored. The time lag distribution curves for copper and aluminum were of the same type, but different from that for nickel. The observations suggest methods for speeding the action of spark plugs and protective gaps, as well as for increasing the reliability of gas‐insulated apparatus. Apparatus operated below the critical conditions can withstand surges which on the average are one thousand times as long as those which would cause the apparatus to fail when operated in the short time lag region. Similarly, a protective gap or spark plug which is to operate on very short surges can be made to break down more consistently by operating at an overvoltage and a gas pressure in the short time lag region.