We present heliocentric radial velocities for a large number of emission lines covering all the optical spectral range for two slit positions of the Galactic H Ii regions M 42 and M 17 and one slit position of M 8. We obtain a fairly similar relation between the heliocentric velocity and the ionization potential of the ions in all the observed slit positions. This general result indicates a similar behaviour of the velocity stratification along the line of sight in the three objects. We suggest that flows of ionized gas streaming away from the molecular clouds toward the observer can explain qualitatively the observations. Based on data obtained at the Observatorio Astronómico Nacional, SPM, B.C., Mexico