Blindfolded rats were trained to stretch across a gap to palpate rough or smooth surfaces with their mystacial vibrissae. Animals learned to discriminate reliably a smooth surface from a rough surface having shallow (approximately 30 microns) grooves spaced at 90 microns intervals. Field-by-field video analyses confirmed that rats used only their vibrissae to contact the discriminanda. The whiskers swept across the surfaces at 696 degrees/sec during forward movements and 1106 degrees/sec for retracting movements. Mean amplitudes, which were 32 degrees, were considerably smaller than the total arc through which whiskers can move. Rats maintained whisker contact with discriminanda for several hundreds of msec, during which time the animals repetitively swept their vibrissae across the surface at a dominant frequency of 8 Hz. The range extended from 1 to 20 Hz, and the frequencies utilized varied within and among subjects. Whiskers contacted the discriminanda along the hair shaft, not at the whisker tips. The hair shafts were bent continually but to varying degrees as an animal palpated the surface, and more than one of the large caudal whiskers were almost always in contact with it. Thus, whiskers are not used independently as rigid levers. Results indicate that the capacity of the rodent whisker system to distinguish a smooth surface from a rough one is comparable to that of primates using their fingertips and suggest common strategies for active touch in the mammalian somatomotor system.