The input/output characteristic of a large aperture (270‐mm diam) electron‐beam‐pumped KrF laser amplifier is obtained for pulses of 6‐ and 40‐ps duration. The off‐resonance absorption coefficient is estimated by measurement of loss at 268 nm. The gain‐to‐loss ratio is found to decrease from 8 to 6 as the specific pump rate is increased from 0.3 to 0.65 MW cm−3. Gain depumping due to amplified spontaneous emission (ASE) is treated by a simple 1D code and found to give good agreement with experiment. A 3D code incorporating wall reflections gives even better agreement. The spatial distribution of gain and ASE is recorded using visible and ultraviolet imaging.