The superconducting properties of high-transition-temperature (high-T c) copper-oxide superconductors depend sensitively on their microstructure. For example, the critical current density Jc of epitaxial YBa2Cu3O7 films can be several orders of magnitude higher than that of fine-grained polycrystalline films and ceramics1,2. Local variations in the size and distribution of features such as grains, grain boundaries, anisotropy and composition play an important role in determining the superconducting properties, and also profoundly affect the normal-state properties. A technique capable of probing superconducting properties with high spatial resolution-smaller than the grain size-is thus important not only for basic understanding, but also for device and microelectronics applications. Here we combine the techniques of micro-patterning3,4 and low-temperature scanning electron microscopy (LTSEM) 5,6 to obtain the first spatially resolved observation of local values of the critical current Ic in high-Tc polycrystalline films. Both the range of values of Ic and the spatial distribution of the local Ic can be determined with a resolution approaching 1 (μm. This new technique can be extended easily to the spatially resolved investigation of Tc distribution, flux pinning and flux flow6. © 1988 Nature Publishing Group.