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The distribution of expansion velocities of the gas of planetary nebulae is compared to that for the circumstellar envelopes of AGB stars. We have examined the relationship between ionized and neutral expansion velocities for the nebulae with detected circumnebular neutral shells. It is found that compact planetary nebulae with ionized radii less than about 0.05 pc show a distribution of expansion velocities that is similar to AGB envelopes, while large planetary nebulae with ionized radii greater than about 0.30 pc show a significantly different expansion velocity distribution than AGB envelopes. Among the large planetary nebula subset, only those nebulae of Greig's class C have an expansion-velocity distribution significantly different than AGB stars, while large Greig's class B nebulae have the same expansion-velocity distribution as AGB stars. The average velocity difference between AGB stars and large Greig's class C nebulae is well predicted by the interacting stellar winds model of planetary nebula evolution. However, the acceleration of ionized gas predicted by the interaction of stellar winds is not evident in the observed expansion-velocity distribution of large Greig's class B nebulae.

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Optical spectra centered near 5000 A have been obtained of the compact planetary nebulae Ml-6, IC 2149, Ml-78, NGC 7027, and CN3-1. The expansion velocities of the ionized outflow and the systemic velocity of each nebula have been determined from the [O III] lines at 5007 A and 4958 Â.