The receptive fields of the sensorimotor cortex neurons identified by electrocutaneous stimulation were modified by microelectrophoretically applied picrotoxin which is known to reduce inhibition. A relatively short application of picrotoxin (90 nA during 3–6 min) markedly increased the size of the neuronal receptive fields in the sensorimotor cortex. Control application of glutamate showed that additional depolarization did not affect receptive fields of the spontaneously active units. Our results together with other work in this field further support the hypothesis that inhibitory processes play a major role in forming functional properties of the cerebral cortex neurons.