  We predict the Tully-Fisher (TF) and surface-brightness--magnitude relation
for disk galaxies at z=3 and discuss the origin of these scaling relations and
their scatter. We show that the variation of the TF relation with redshift can
be a potentially powerful discriminator of galaxy-formation models. In
particular, the TF relation at high redshift might be used to break parameter
degeneracies among galactosynthesis models at z=0, as well as to constrain the
redshift distribution of collapsing dark-matter halos, the star-formation
history and baryon fraction in the disk, and the distribution of halo spins.
