Muscle fiber conduction velocity (MFCV) in biceps brachii was studied in traumatic brachial plexus lesions (16 patients) and amyotrophic lateral sclerosis (ALS) (22 patients) by means of an invasive (I-MFCV) and a surface (S-MFCV) method. After complete denervation an exponential decrease of the mean I-MFCV was found, T1/2 = 1.1 month. After 4-5 months this resulted in severely reduced conduction velocities (mean 1.4 m/sec, range from 0.5 to 2 m/sec). Simultaneous with signs of reinnervation, fibers with faster conduction velocities were seen. In ALS, a decrease of the mean I-MFCV was found, and slow conducting fibers were found in every patient on at least one side. At the same time muscle fibers with increased I-MFCVs were found. This increased range of velocities seems based on a combination of slow conducting, atrophic fibers, with fast conducting, hypertrophic fibers, compensating the force loss. In some subjects we found these disturbances without clear abnormalities with concentric needle EMG, and with unimpaired muscle force. The surface EMG measurements in the ALS patients revealed increased S-MFCV values in combination with a decrease of the median frequency (Fmed). We suggest that the opposite finding of an increased S-MFCV is a consequence of the muscle fiber hypertrophy in the surviving, voluntarily recruited, motor units. The simultaneous decrease of the Fmed seems to be caused mainly by the change in shape of the motor unit potential.