Ischemic lesions were produced in the cat visual cortex and changes of responses to visual stimuli were studied in single cells with extracellular recordings. The spontaneous and the visually driven activity were quantitatively analyzed and compared with those in normal controls. While the normal control cells showed a mean response strength of 52.7 impulses/s, this value was increased to 69.3 impulses/s in the vicinity of ischemic lesions. The spontaneous activity was also increased. Epileptiform burst activity with frequencies above 500 I/s was observed in 37.7% of the cells. Directional and orientational specificity was significantly reduced in these regions because of an increase in responses to stimuli in non-preferred directions and with non-preferred orientations. Thus, an increase in activity-dependent energy consumption occurs in the vicinity of lesions in the ischemically severed cortex.