A pulsed transversely excited CO2 laser operating on the 9R(30) transition is used to optically pump mixtures of NH3 in buffer gas. A simple oscillator/amplifier system characterizes the performance of the NH3 amplifier in the 11 μm region. Small-signal gain coefficients of >10%/cm measured on the aQ(3,3) transition at 10.7 μm, while pump conversion efficiencies of ∼50% are shown to occur under saturation conditions. The NH3 laser system is described by a rate-equation model, which is validated by comparison with experiment over a wide range of operating conditions. Measurements are made for NH3 concentrations ranging from 0.05 to 0.2%, with Ar, N2, and He buffer gas pressures from 170 to 700 Torr, and for gas temperatures from 200 to 300 K. Optically pumped NH3 is shown to be a versatile and efficient system for the amplification of mid-infrared radiation.