We report the results of a systematic study on the spatial variation of the electron density in a selected group of bright giant H II complexes in the galaxies M 101, M 51, and NGC 6822. Density values for the ionized gas of the regions have been obtained from the forbidden S II 6717/6731 ratio, using long-slit spectrophotometry at high spectral and spatial resolution. Our study shows evidence that some of the regions present gradients in the distribution of the ionized gas, the brightest zones being associated with the maximum electron density values. For those cases where density variation is defined, different models that describe the internal gas density distribution have been considered and compared with the observations. We conclude that more realistic theoretical models of nebulae should take into account the internal density gradients within the regions.