Rats learned to discriminate between two degrees of roughness, detecting the stimuli only with their vibrissal system. After bilateral trimming of the vibrissae, performance abruptly dropped to chance levels and remained there throughout the period in which the vibrissae were kept trimmed. After the vibrissae regrew to approximately normal lengths, rats again exhibited high performance levels and demonstrated retention of the task. This discriminative procedure would appear to be appropriate for psychophysical assessment of vibrissal system function.