  We present a theoretical study of the orbital magnetoresistance in a unixial
anisotropic metal within the relaxation-time approximation. The appearance of a
new dimensionless scale, delta=4t_perp/epsilon_F, allows the possibility of a
new region at intermediate fields where the magnetoresistance is linear in
applied magnetic field for currents flowing along the unixial direction. (Here,
t_perp characterizes the bandwidth along the unixial direction.) In the limit
of large anisotropy (small delta), corresponding to a quasi-two-dimensional
metal made up of weakly coupled layers, we obtain an analytic expression for
the magnetoresistance valid for all magnetic fields. We test our analytic
results numerically and we compare our expressions with the c-axis
magnetoresistance of Sr_2RuO_4.
