In rats, whisker trimming during development leads to persistent alterations in the function of cortical barrel circuits and to behavioral deficits later in life. Here we examined how whisker trimming begun either at birth (P0) or on postnatal day 12 (P12), around the onset of whisking behavior, affects receptive fields of layer IV barrel neurons. All whiskers on the left face were trimmed for 40-45 days and then allowed to regrow fully. Extracellular single-unit recordings and controlled deflections of principal and adjacent whiskers (PW and AW, respectively), individually or in paired combinations, were used to assess excitatory and suppressive effects of neighboring whiskers on barrel neurons. Results indicate that whisker trimming both from P0 and P12 leads to enlarged excitatory and weakened inhibitory receptive fields in layer IV neurons. PW- and AW-evoked responses are larger in magnitude in trimmed than in control animals; AW-evoked responses are disproportionately affected, decreasing the spatial focus of barrel neurons. Deprivation after P12 accounts for approximately 50% of the total effect observed in P0 trimmed animals. Suppressive interactions, evoked by two whiskers deflected in succession, are weaker in trimmed than in control animals. Suppressive caudal/rostral and ventral/dorsal gradients, however, seem unaffected by sensory deprivation. Thus the developmental period during which experience persistently modifies maturing barrel circuitry extends up to and likely beyond the onset of whisking behavior. Sensory deprivation during this time affects development of both excitatory and inhibitory receptive fields of barrel neurons and likely impairs cortical integration of sensory information from multiple whiskers.