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When astronomers study the dark matter halos of spiral galaxies, theynormally assume that the disk mass-to-light ratio is *constant*. We describe amethod of analyzing the kinematics of planetary nebulae (PNe) in nearby face-onspiral galaxies to test this assumption. Since the restoring force for stellarmotions perpendicular to the galactic disk is proportional to the disk masssurface density, measurements of the vertical velocity dispersion can be usedto produce an independent measure of the total amount of matter in the disk.Our steps are: (1) to identify a population of PNe by imaging the host spiralin several filters, and (2) to isolate the vertical velocity dispersion fromspectroscopic observations of the PNe. Our first results for the PNe of M33indicate that the mass-to-light ratio of the galaxy's disk actually *increases*by more than a factor of 5 over the inner 6 disk scale lengths. We have begunsimilar studies of the PNe in five more face-on galaxies: M83, M101, M94, NGC6946, and M74. These data will also produce additional science such as galaxydistances and constraints on the disk transparency.Comment: 4 pages, 4 figures, for "Planetary Nebulae as Astronomical Tools"  Conference Proceedings (in Gdansk, Poland June 28-July 2, 2005

Planetary Nebula Studies of Face-On Spiral Galaxies: Is the Disk Mass-to-Light Ratio Constant?