A travelling-wave optical parametric amplifier (OPA) is treated, and basic theoretical results are summarized. An OPA connected to KDP crystals pumped by second harmonic pulses (λ= 0·532 μm, τ ≃ 20 ps) from a Nd: YAG laser was studied. The dependence of temporal and spectral parametric pulse properties on the conversion rate (gain saturation) was investigated. Nearly bandwidth-limited pulses (τ Δv˜ 0·6) were obtained at low energy conversion. A pulse reduction to 8 ps was measured in this case. At higher conversion ≳ 10 per cent, the stability of the output energy was higher. However, the pulse quality decreased considerably.