In this experiment one utilizes a microwave confocal open resonator in the transmission configuration, having a Q factor of 5 105. The resonator is filled with ammonia gas at the pressure of 1 mTorr. A klystron oscillator, frequency stabilized on the 3-3 ammonia inversion line, feeds the resonator which is tuned at the same frequency. In these experimental conditions, if the input power PI is continuously increased from a low to a sufficiently high value, the output power changes abruptly from a low to a high value. For a fixed gas pressure, this change occurs at a fixed threshold input power Pup. The same occurs if the input power is continuously decreased from a high value; in this case one has Pdown< Pup, Pdownbeing the threshold power at which the system switches from the high to the low transmission state.