To determine the diagnostic and prognostic significance of evoked spinal potentials (ESPs), reproducible, compressive spinal cord lesions were produced in dogs under ESP monitoring. Both motor recovery and histologic findings were investigated in relation to the amplitude and persistence of the ESPs. Animals that had moderate, anterior compression (51% of canal width) in which the ESP was maintained had considerable motor recovery. Among animals that had submaximal compression (60%), motor recovery was good in those with persistent ESPs. The compression ratio at which ESPs terminated (67%) was termed the "electrophysiologic maximum tolerable compression." Continuous ESP expiration resulted in irreversible lesions and loss of motor recovery. Compressive spinal cord lesions of reproducible, graded severity were employed for the first time in our study of ESP. Persistence and amplitude were both useful in predicting not only the severity and extent of this type of lesion but also the recovery.