  In the last 20 years, rotation curves derived from H I kinematics obtained on
radio synthesis instruments were used to probe the dark matter distribution in
spiral and dwarf irregular galaxies. It is shown, with the aid of the Sd galaxy
NGC 5585, that high resolution 2--D H II kinematics is necessary to determine
accurately the mass distribution of spirals. New CFHT Fabry--Perot Hff
observations are combined with low resolution Westerbork H I data to study its
mass distribution. Using the combined rotation curve and best fit models, it
can be seen that the (M/LB) of the luminous disk goes from 0.3, using only the
H I rotation curve, to 0.8, using both the optical and the radio data. This
reduces the dark--to--luminous mass ratio by 30%.
