Palmar stimulation was used to assess median nerve conduction across the carpal tunnel. In 50 hands from 25 control subjects, motor and sensory latencies in the wrist-to-palm segment (mean +/- SD: 1.15 +/- 0.21 msec and 1.12 +/- 0.21 msec respectively) were less than half the conventional terminal latencies in the wrist-to-muscle and wrist-to-digit segment (3.01 +/- 0.44 msec and 2.47 +/- 0.39 msec). Motor and sensory conduction velocities (MNCV and SNCV) in the wrist-to-palm segment (56.0 +/- 7.6 m/sec and 58.7 +/- 7.5 m/sec respectively) were comparable to those in the elbow-to-wrist segment (57.0 +/- 4.5 m/sec and 62.4 +/- 5.7 m/sec). In 20 symptomatic hands from 13 patients with mild carpal tunnel syndrome, delay in motor and sensory terminal latencies (3.91 +/- 0.67 msec and 2.90 +/- 0.57 msec) was primarily attributable to increased conduction time in the wrist-to-palm segment (1.96 +/- 0.59 msec and 1.58 +/- 0.49 msec) and not in the remaining more distal portions. Consequently, MNCV and SNCV were significantly (P less than 0.001) slowed when calculated in the segment across the carpal tunnel (36.6 +/- 11.2 m/sec and 44.9 +/- 11.8 m/sec), even though the conventional terminal latencies from the stimulus site at the wrist were often within normal limits.