  We discuss the potential of the Sudbury Neutrino Observatory (SNO) to
constraint the four--neutrino mixing schemes favoured by the results of all
neutrino oscillations experiments. Our results show that some information on
the value of $\cos^2(\vartheta_{23}) \cos^2(\vartheta_{24})$ can be obtained by
the first SNO measurement of the CC ratio, while considerable improvement on
the knowledge of this mixing will be achievable after the measurement of the
NC/CC ratio.
