A vibrating microelectrode, or vibrating probe (Jaffe and Nuccitelli, 1974), was used to map the pattern of artificially created electric currents flowing around single muscle fibers at the edge of frog cutaneous pectoris muscles. When a muscle fiber was impaled with a micropipette, a "point sink" of current was often created at the site of impalement because of injury to the cell membrane. Current, being drawn from the flanking membrane, flowed into the cell only at this point. This defined current allowed us to map the spatial resolving power of the vibrating probe by moving to different positions near the impalement site. The results suggest that under our experimental conditions the limit of resolution is a few tens of micrometers. The results were fit reasonably well by a computer model. Current was also passed through a micropipette and mapped at various positions with the vibrating probe. In this case, the current flowed to a remote reference electrode. With the current electrode in the extracellular fluid, the probe signal decayed as the inverse square of the distance, as expected. With the current electrode placed intracellularly, current was funneled along the muscle fiber axis, reflecting its cable-like properties. The signal recorded by the vibrating probe was altered accordingly, and the results could be well fit by a simple model.