In sensory substitution, information acquired with one sensory modality is use d to accomplish a task which is normally subserved primarily by another sensory modality. We used PET to study cross-modal plasticity in the congenitally blind , using electrotactile stimulation of the tongue. Blind (n=6) and sighted blindf olded controls (n =5) were scanned before and after they were trained to use the ir tongue in a Snellen orientation detection task. Results showed that both grou ps of subjects learned the discrimination orientation task after seven 1 h train ing sessions. Before training, no significant changes in regional cerebral blood flow (rCBF) were observed in the occipital cortex in either group. In sharp con trast, activity in the occipital cortex increased after practice for the blind, but not for the sighted, providing evidence for training-induced plasticity in the blind. An inter-regional correlation analysis showed that task-related rCB F changes in left posterior parietal cortex were positively correlated with rCBF changes in the occipital area of the trained blind. These data reveal that cros s-modal plasticity in the blind develops rapidly and that the occipital cortex is part of a functional neural network for tactile discrimination in conjunction with the posterior parietal cortex. Our data further show that the tongue can a ct as a portal to convey somatosensory information to visual cortex. In sensory substitution, information acquired with one sensory modality is use d to accomplish a task which is normally subserved in by another sensory modality. We used PET to study cross-modal plasticity in the congenitally blind, using electrotactile stimulation of the tongue. Blind ( n = 6) and sighted blindf olded controls (n = 5) were scanned before and after they were trained to use the ir tongue in a Snellen orientation detection task. Results showed that both grou grou ps of subjects learned the discrimination orientation task after seven 1 h training ing sessions. Before training, no significant changes in regional cerebral blood flow (rCBF) were observed in the occipital cortex in either group. In sharp con trast, activity in the occipital cortex increased after practice for the blind, but not for the sighted, providing evidence for training-induced plasticity in the blind. An inter-regional correlation analysis showed that task-related rCB F changes in left posterior parietal c ortex were positively correlated with rCBF changes in the occipital area of ​​the trained blind. These data reveal that cros s-modal plasticity in the blind develops rapidly and that the occipital cortex is part of a functional neural network for tactile discrimination in conjunction with the posterior parietal cortex. Our data further show that the tongue can a ct as a portal to tell somatosensory information to visual cortex.