We present radial velocities of 33 red giants in the Leo I dSph galaxy obtained from spectra taken with the HIRES echelle spectrograph on the Keck I telescope .
These data have a mean precision of 2.2 km s ^ { -1 } and lead to estimates of the central velocity dispersion and systemic velocity of Leo I of 8.8 \pm 1.3 km s ^ { -1 } , and 287.0 \pm 1.9 km s ^ { -1 } , respectively .
The systemic velocity confirms past results that Leo I has an unusually large galactocentric velocity , implying the presence of a massive dark halo in the Milky Way or an extended dark component pervading the Local Group .
The V-band ( M / L ) ratio of Leo I is in the range 3.5-5.6 .
We have produced a set of models that account for the effects of stellar evolution on the global mass-to-light ratio of a composite population .
Because Leo I contains a dominant intermediate-age population , we find that the V-band mass-to-light ratio of Leo I would be in the range 6-13 if it were composed exclusively of old stars such as found in globular clusters .
This suggests that Leo I probably does contain a significant dark halo .
The mass of this halo is approximately 2 \times 10 ^ { 7 } M _ { \odot } , similar to the dark halo masses inferred for all other Galactic dSph galaxies .

Because Leo I is isolated and has passed the Milky Way at most once in the past , external tides could not plausibly have inflated its central dispersion to the observed value .
We also considered whether MOdified Newtonian Dynamics ( MOND ) could account for the internal kinematics of Leo I and conclude that this alternative gravitational model can account for the Leo I kinematics adequately without requiring a dark halo .
The agreement with MOND is particularly good if the velocity dispersion exhibits some anisotropy or the underlying stellar mass function is only slightly different than a classical Salpeter law .