Immunocytochemical techniques were used to study the effects of tactual deprivation on glutamic acid decarboxylase (GAD) containing neurons in rat somatosensory barrel cortex. In normal rats GAD immunoreactive neurons and puncta are present in all laminae, with dense patches of GAD immunoreactive puncta centered on the barrels in lamina IV. Trimming whiskers of adult rats leads to a reversible decrease of GAD immunoreactivity in barrels corresponding to trimmed hairs. Intensity of GAD staining also is reversibly altered in supragranular laminae of nondeprived barrel columns flanked by deprived barrels. This indicates that GAD levels in the barrel cortex ordinarily fluctuate with changes in sensory input. By contrast, animals whose whiskers are trimmed from birth have normal GAD staining in both deprived and nondeprived barrels. Moreover, if trimmed whiskers of neonatally deprived animals are allowed to grow to normal lengths and are retrimmed later in adulthood GAD staining is not affected. Thus early tactual deprivation disrupts mechanisms that permit modulation of transmitter enzyme levels in cortical neurons following changes in sensory experience.