The forbidden O III expansion velocities for 22 planetary nebulae (PN) were derived from high-dispersion spectra taken at the Astrophysical Observatory of Asiago. The sample contains objects in very different evolutionary phases: compact nebulae with high surface brightness, nebulae with intermediate radii, and faint nebulae close to the optical limit. These new data, along with those reported in the literature, indicate that PN of the class C of Grieg (1971, 1972) are ejected and excited by post-AGB stars having a final mass of about 0.60 solar masses or less, while PN of class B have central stars more massive than 0.60 solar mass. The expansion velocity versus radius relations for both types of PN can be understood in terms of the evolutionary model suggested by Sabbadin et al. (1984).