The origin of the luminescence of InGaN/GaN multiple quantum-wells has been studied at high optical excitation by means of time-resolved luminescence technique combined with the variable-stripe method. Spontaneous transitions from localized states and stimulated emission of free carriers both from the quantum well and the barrier or buffer layers have been identified. The estimated room-temperature optical gain coefficient of 8000 cm(-1) was found to be close to the theoretical value.