Progression of the neuropathy in the Twitcher mouse (twi-C57BL/6J), an animal model of globoid cell leukodystrophy, was assessed with serial motor nerve conduction studies from just after birth until near death (day 45) and after hematopoietic cell transplantation (HCT). Under ether anesthesia, the tibial nerve was stimulated percutaneously at the sacral notch and at the ankle, and recordings were made from plantar foot muscles. Motor conduction velocity (MCV), distal latency, amplitude, duration and number of phases of compound muscle action potentials on proximal (pCMAP) and distal (dCMAP) stimulation were measured. In 15-19 day-old Twitcher, despite the absence of motor signs, MCV was significantly reduced, 12.8 +/- 2.8 (10) m/s (M +/- SD, No. of recordings), compared with unaffected siblings, 18.1 +/- 2.6 (21) m/s (P less than 0.01). The ratio of pCMAP to dCMAP amplitudes was reduced in the Twitcher, 0.39 +/- 0.13 (10), compared with controls 0.72 +/- 0.17 (21) and the ratio of pCMAP to dCMAP phases was increased (2.8 +/- 0.8 (10) vs 1.0 +/- 0.2 (21), P less than 0.01 for all). As neurologic signs progressed by 35-39 days, MCV became slower, 5.8 +/- 1.0 (11) m/s, pCMAP and dCMAP became smaller, but the ratio of pCMAP to dCMAP amplitudes in the Twitcher (0.55 +/- 0.36, 11) was similar to controls (0.71 +/- 1.0, 20) as was the ratio of pCMAP to dCMAP phases (1.0 +/- 0.4 vs 1.0 +/- 0.1). These results suggest that there is diffuse non-uniform slowing of nerve conduction with block especially in proximal nerve fibers initially. With HCT, mean MCV remained slow (6.7 +/- 1.2 (18) m/s, vs 34.5 +/- 3.9 (12) m/s) but motor function persisted.