The temperature and magnetic field dependences of the critical-current density of superconducting YBa2Cu3O7 films have been determined from transport measurements across individual  tilt boundaries and within single  grains. The results provide evidence that the basal-plane Jf is limited by flux creep. In contrast, the I-V characteristics and the temperature magnetic field dependences of the grain-boundary Jc are consistent with a superconductor- normal-superconductor-type weak-link behavior. These results imply that the transport properties of these materials can be treated within a BCS framework. © 1988 The American Physical Society.