Rats explore their surroundings through rhythmic movement of their mystacial vibrissae. At any given moment, multiple whiskers are simultaneously moved and may contact the surface of an object. The aim of this work is to understand how simultaneous multiple-whisker deflections are processed in the somatosensory cortex. Arrays of 25 electrodes were inserted into the vibrissal representation of barrel cortex of adult rats. Multi-unit responses were recorded during (i) stimulation of single whiskers, and (ii) simultaneous stimulation of two, three or four whiskers of a whisker arc or whisker row. The whole-array response elicited by the simultaneous stimulation of multiple-whiskers (observed response) was compared to a multiple-whisker response predictor, defined as the sum of the whole-array responses to the separate stimulation of the corresponding single whiskers. The observed response to stimulation of four whiskers was nearly always less than the predicted response, indicating a sublinear summation of multiple coincident inputs. Examining the poststimulus time course of sublinearity, we found that the earliest cortical response to whisker deflection - reflecting the thalamocortical volley - was linear, whereas the successive cortical response was highly sublinear. This suggests a cortical origin of the phenomenon.