This paper reports the results of investigations carried out on the influence of simmer current on flashlamp impedance and performance of a flashlamp-pumped Nd:YAG laser operating in the quasi-CW mode. A pulse power source with an adjustable output pulsewidth (1-20 ms) and an adjustable simmer current source (30 mA to 6 A) developed specifically for the study were used. It is found that the simmering of the lamp reduces the impedance factor of the lamp, increases the correct arc diameter, and reduces the peak current density of the flashlamp discharge. It improves the efficiency of the flashlamp pump source and, hence, the efficiency of the Nd:YAG laser also. An appropriate choice of the simmer current, using the results obtained, can lead to higher efficiency of the laser and increased life of the lamp.