OBJECTIVE We studied sequential waveform changes associated with a progressive conduction block to elucidate the relative vulnerability of slow versus fast conducting fibers to a focal compression. METHODS In 12 healthy men, orthodromic compound sensory nerve action potentials (SNAPs) of the median nerve were recorded unipolarly at 4 sites over the forearm during a 30 min period of constant, localized compression of the nerve at the wrist. RESULTS Initial findings at the compression site consisted of nearly immediate reduction in size of the negative component accompanied by progressive enlargement of the initial-positive component. Recording at 2 cm proximal to the compression and at the elbow showed no change in onset latency initially, indicating at least partial preservation of the fast conducting fibers. Amplitude also remained unchanged for about 20 min, presumably because loss of negative and positive peaks compensated each other until conduction block began to involve a greater number of the fast conducting fibers. CONCLUSIONS The analysis of waveform changes and their time course suggests that a focal compression initially affects the slow conducting small diameter fibers. Partial conduction block gives rise to complex waveform changes depending on recording sites. A reduction in one polarity of constituent nerve fiber action potentials may enhance the other polarity of the SNAP.