Excitation of electron plasma waves by the mixing of two microwave bea
ms is studied in a laboratory plasma. The first experimental observati
on of pump wave depletion is presented. The numerical calculations, ba
sed on one-dimensional model, describing space-time evolution of eight
interacting waves, are in good agreement with experimental data. Esti
mation of the depletion on the basis of conservation laws is also disc
ussed.