We have studied electrophysiologically by single cell recording in the visual cortex, whether modification of the visual system in developing and in adult cats by hydrocephalus has an effect on processing of visual information. One of our cats (H1) had developed a complete hydrocephalus and the others partial, as proved by either complete or partial dilation of the lateral ventricles, respectively and by the thinning of the cortex. Despite this, the horizontal lamination and the vertical organization of the cortex were fully preserved. Except for the optic radiation and the corpus callosum which was remarkably modified, the optic tract, chiasm, nerve and retina were morphologically and histologically normal. The visual behavior of the hydrocephalic cats was normal. This was also reflected, by and large, in the physiological properties of the visual cortex. However, in cat H1 there were many more visually unresponsive cortical cells in comparison to its matched controls (C1) and the normal cats. A reduced responsiveness was also found in cat H2 with partial hydrocephalus but not in the other partial hydrocephalic cats. Similarly, the ocular dominance distribution of the cells was affected in cat H1 in comparison to the control cats as indicated by the changes found in the relative proportions of contralaterally and ipsilaterally driven cells in the two hemispheres. No change was, however, found in the partially hydrocephalic cats. Most of the cells in the hydrocephalic cats were orientation specific, similarly to the result of their matched controls. Direction specific cells were much smaller in proportion in cat H1 but not in the other cats, in comparison with their matched controls. In keeping with this, a large increase was found in the receptive field area of cat H1, a smaller one in cat H2 and none in the other hydrocephalic cats in comparison to the matched controls. The eccentricity distribution of the receptive fields in the hydrocephalic cats was the same as expected under normal conditions. It was concluded that in the way hydrocephalus had modified the brain of several of our cats, a quantitative effect was induced in visual cortex cells leading to some degradation of function; this change, however, did not interfere with their basic visual properties.