We present the results of the investigation of a Nd:glass rod laser amplifier 320 mm long and 150 mm in diameter. The gain and depolarization distributions over the amplifier aperture as well as the stored energy have been measured for different values of the pump energy. The small signal gain averaged over the rod aperture was 2.3 when the pump energy was 48 kJ. The corresponding stored energy was 500 J. We have determined the maximal pulse repetition rate that is quiet safe from the point of the thermal shock damage. It is about 1 shot per 5 minutes. The calculated focal length of the thermal lens exceeds 1.5 km in such a regime. We have shown that the use of a couple of such amplifiers gives the possibility to increase the energy of laser pulses up to 650 J.