We present an experimental study of the dynamical screening effect of the built-in field in GaN-based quantum well structures by means of picosecond time-resolved luminescence measurements. Our results clearly show that the recombination dynamics of the photoexcited carriers is influenced by two main effects: a) the charge accumulation in the wells caused by the separation of the wave-functions and the increase of the radiative recombination time due to the polarization field, b) the loss of carriers from the ground level induced by the recombination processes. The experimental results are compared with theoretical predictions based on a simultaneous self-consistent solution of a rate-equation for the time evolution of the carrier population together with the Schrodinger and Poisson equations.