The performance of a ruby laser amplifier is described and compared with a theoretical model. The ruby laser amplifier was driven by a Q-switched laser oscillator with a stable output, whose properties are due, in part, to a newly developed output coupler and a spinning roof-top prism for Q-spoiling. From the theoretical amplifier model, a relation between the population inversion and the energy stored in the system supplying the xenon flash lamps is derived and fitted to the particular laser configuration by a single parameter established by the laser threshold. Experimentally, the gain of the 15-cm (6-in.) ruby amplifier saturates at values between 5 and 7, apparently caused by complete pumping of the system rather than limited by the energy input to the flashlamp pump.