  The importance of the distribution and ``weakness'' of grain boundary
junctions in the magnetic field dependence of the transport critical current in
HTc films is assessed through simulations. The system is studied with the
applied field either parallel or perpendicular to the c axis of the sample. For
realistic sample parameters, it is demonstrated that the presence of ``high''
misorientation angles between grains depresses the zero - field critical
current density in both orientations, and provokes a transition from
pinning-mediated to Fraunhoffer-like field dependencies of the critical current
density. Our results also suggest that there is a threshold misorientation
angle above which the critical current density remains constant.
