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We use the tapering effect of Halpha/[NII] rotation curves of spiral galaxiesfirst noted by Goad & Roberts (1981) to investigate the internal extinction indisks. The scale length of exponential fits to the inner part of rotationcurves depends strongly on the disk axial ratio. Preliminary modelling of theeffect implies substantial opacity of the central parts of disks at awavelength of 0.66 \mu. In addition, the average kinematic scale length ofrotation curves, when corrected to face--on perspective, has a nearly constantvalue of about 1.7 h^{-1} kpc, for all luminosity classes. The interpretationof that effect, as the result of the increasing dominance of the baryonic massin the inner parts of galaxies, yields a mean baryonic mass--to--light ratio inthe I band \Upsilon_I= 2.7h M_sun/L_{sun,I}, within the inner 1.7 h^{-1} kpc ofdisks.Comment: 11 pages, including one table and one figure, AAStex scheduled to  appear in the Astrophysical Journal (Letters

The Inner Scale Length of Spiral Galaxy Rotation Curves