1. Microelectrodes and controlled stimulation of mystacial vibrissae were used to examine the response properties of thalamic barreloid neurons in adult rats that had been raised in an abnormal tactile environment produced by having one (row C) or four (all but C) rows of whiskers trimmed to the skin surface from birth to 45-53 days of age. Whiskers were allowed to regrow for an average of approximately 9 weeks before electrophysiological study. 2. Spontaneous and stimulus-evoked activity of single neurons in deprived barreloids was similar to that observed in control animals except that activity levels following stimulus offsets were slightly elevated. These effects were more pronounced in nondeprived barreloids. In addition, neurons in nondeprived barreloids responded more vigorously during the stimulus plateau than neurons in deprived or control barreloids. Responses to stimulus onsets and offsets were statistically equivalent among deprived, non-deprived, and control barreloid neurons. 3. The findings indicate that increased spontaneous and stimulus-evoked activities observed previously in deprived cortical barrels reflect abnormalities within the cortex itself. Previously observed increases in neuronal activity in nondeprived cortical barrels probably reflect abnormalities in the input signals they receive from nondeprived barreloids as well as possible changes in cortical circuitry. Effects in the thalamus may be due to abnormal lateral inhibitory interactions between inputs from previously trimmed and nontrimmed whiskers.