We present a detailed investigation of the exciton and free-carrier dynamics in high quality GaN/ AlGaN MBE-grown quantum wells under different experimental conditions. In particular time-resolved photoluminescence spectra clearly indicate a major role played by the photoinduced screening of the internal electric field in the carrier recombination. Resonant and non-resonant time-resolved spectra are compared, suggesting a more efficient screening of the field when the carriers are created in the well. Experimental data are analyzed in the framework of a model based on the self-consistent solution of both Schrodinger and Poisson equations and a rate-equation for the time evolution of the carrier dynamics.