The effect of pairing two vibrissa stimulations on unit responses of the barrel field of the somatosensory cortex were studied in partially restrained but awake and undrugged rats. Before pairing, one of the stimulations (S2) evoked a stable, short-latency and excitatory response from the recorded unit. Depending on the neuron, the other stimulation (S1), preceding S2 by 500 ms, did or did not have an effect before pairing. In a number of cases, the S1-S2 association produced significant changes in the unit responses: (1) the appearance of an excitatory response to S1 when that stimulus was ineffective before pairing; (2) the modification of pre-existing responses to S1 and/or S2. In all instances these modifications consisted in the decrease or disappearance of the "afferent inhibition" and/or the appearance of long-latency excitatory components. These effects appeared after some 30-100 trials and persisted in some cases up to 20 min after interruption of pairing. Our observations provide the first physiological data on the plasticity of the vibrissa projections in the chronic adult rodent. Though the underlying plastic neural elements and mechanisms remain to be specified, these phenomena suggest that "learned" changes in unit activity may occur in sensory systems and not only in "non-specific" ones.