  Observations show that the slope of the inner part of the H$\alpha$/[NII]
rotation curves of disk galaxies is depressed by extinction: at fixed
luminosity, the observed slope is in fact seen to depend on the disk
inclination to the line of sight. Using a simple extinction model, we are able
to reproduce well the observed trends. The model assumes an exponential
distribution, both in the radial and $z$ directions, identical for
star--forming regions and dust. Fits to the data are optimized by varying the
scale height and scale length of absorbers, as well as the disk's central
optical depth $\tau_\circ$, as seen face--on. The observations indicate that
disk central opacity increases with total luminosity. Model fits for the most
luminous galaxies (brighter than $M-5\log h=-21.4$ in the I band) yield
$\tau_\circ \simeq 3.5^{+4.0}_{-2.0}$, near $\lambda=0.66$ $\mu$m. The large
uncertainty on the inferred value of $\tau_\circ$ is due to the poorly known
characteristics of the distribution of absorbers in the disk, as well as to the
likelihood of strong departures from an exponential radial distribution near
the galaxy centers.
