  We study the transport properties of the two-dimensional electron gas in
AlGaAs/GaAs heterostructures in parallel to the interface magnetic fields at
low temperatures. The magnetoresistance in the metallic phase is found to be
positive and weakly anisotropic with respect to the orientation of the in-plane
magnetic field and the current through the sample. At low electron densities
($n_s< 5\times 10^{10}$ cm$^{-2}$) the experimental data can be described
adequately within spin-related approach while at high $n_s$ the
magnetoresistance mechanism changes as inferred from $n_s$-independence of the
normalized magnetoresistance.
