  Updating our previous measurements with new data and analysis modifications,
we report preliminary results on the energy-dependent deficit of muon neutrinos
from the Fermilab NuMI beam as observed with the MINOS Far Detector located 735
km away in the Soudan Underground Laboratory. From an exposure of 2.50x10^20
protons on target, we observe 563 charged-current nu_mu interaction candidates
in the Far Detector, where 738+/-30 events are expected in the absence of
neutrino oscillations. We have analyzed these data assuming two-flavor nu_mu to
nu_tau oscillations. From a simultaneous fit to the reconstructed nu_mu energy
spectra obtained during two running periods we obtain the neutrino squared-mass
difference Delta m_{32}^2 = (2.38 +0.20/-0.16) x 10^-3 eV^2/c^4 with errors at
68% confidence level (CL), and mixing angle sin^2(2 theta_23) > 0.84 at 90% CL.
The uncertainties and confidence intervals include both statistical and
systematic errors. All results and plots presented here are preliminary.
