The redistribution of neuronal activity across rat barrel cortex following an alteration in whisker usage has been investigated. In adult rats, two mystacial vibrissae - D(2) and one neighbor, D(1) or D(3) - were left intact while all other vibrissae on that side of the snout were clipped. Neurons in contralateral barrel cortex were sampled with a microelectrode array 3.5 days later. Stimulation of clipped vibrissae produced a narrow spatial distribution of cortical activity, whereas stimulation of intact vibrissae produced a widened spatial distribution. Simultaneous recordings from multiple cortical barrel-columns suggest that changes in the effective connectivity between barrel-columns may partially account for this redistribution of sensory responses. Evidence is also presented for a second mechanism, a release from inhibition in sensory-deprived cortical areas. A model is therefore proposed where these two mechanisms operate together to regulate the cortical distribution of evoked activity.