Peripheral nerve fascicles are deliberately impaled during microneurography experiments. We have assessed conduction block under these circumstances. First we recorded compound action potentials (CAP) in the sural nerve following stimulation of the sciatic nerve. The insertion of injection and microneurography electrodes between stimulation and recording sites produced a 20% decrease in the size of the CAP, which was maintained for the 10 min duration of impalement. After withdrawal, the conduction block partially resolved. In other experiments, single-unit action potentials were recorded from L4 and L5 dorsal roots following peripheral nerve stimulation. Microneurography electrodes inserted into the sciatic nerve produced conduction block in 50% of these axons. When axons were blocked, anodal stimulation through the tungsten electrode became more effective than cathodal stimulation. These results suggest that a temporary conduction block occurs in a significant number of myelinated fibers near the site of an inserted electrode.