  Experiments strongly suggest that the flavour mixing responsible for the
atmospheric neutrino anomaly is very close to being maximal. Thus, it is of
great theoretical as well as experimental importance to measure any possible
deviation from maximality. In this context, we reexamine the effects of matter
interactions in long baseline neutrino oscillation experiments. Contrary to
popular belief, the muon neutrino survival probability is shown to be quite
sensitive to matter effects. Moreover, for moderately long baselines, the
difference between the survival probilities for $\nu_\mu$ and $\bar\nu_\mu$ is
shown to be large and sensitive to the deviation of $|U_{\mu 3}|$ from
maximality. Performing a realistic analysis, we demonstrate that a muon-storage
ring $\nu$-source alongwith an iron calorimeter detector can measure such
deviations. (Contrary to recent claims, this is not so for the NuMI--{\sc
minos} experiment.) We also discuss the possible correlation in measuring
$U_{\mu 3}$ and $U_{e3}$ in such experiment.
