The electrical activity of numerous electroceramics originates with space charge potential across internal interfaces. Space charge formation and the resultant potential barrier at interfaces are believed to be responsible for many interesting and useful properties of electroceramics, ranging from nonlinear current–voltage characteristics to enhanced dielectric properties. Direct current electrical measurements of individual grain boundaries in Nb donor doped SrTiO3 bicrystals reveal a highly resistive and nonlinear behavior compared to single crystals. The origin of this nonlinear resistance has been examined with electron holography, observing both static and dynamic attributes of the internal potential. In the static case with no applied current, the grain boundary potential barrier height was measured to be about 0.45 V. During the application of a high current, this potential barrier was suppressed, presenting the first direct real space evidence for breakdown of an internal grain boundary barrier. © 1999 American Institute of Physics.