Recordings were made from single cells in the striate cortex of lightly anaesthetized cats. The retinas were stimulated separately or simultaneously with light spots of various sizes and shapes. In the light-adapted state cortical cells were active in the absence of additional light stimulation. Increasing the depth of anaesthesia tended to suppress this maintained activity. Restricted retinal areas which on illumination influenced the firing of single cortical units were called receptive fields. These fields were usually subdivided into mutually antagonistic excitatory and inhibitory regions. A light stimulus (approximately 1 s duration) covering the whole receptive field, or diffuse illumination of the whole retina, was relatively ineffective in driving most units, owing to mutual antagonism between excitatory and inhibitory regions. Excitatory and inhibitory regions, as mapped by stationary stimuli, were arranged within a receptive field in a side-by-side fashion with a central area of one type flanked by antagonistic areas. The centres of receptive fields could be either excitatory or inhibitory. The flanks were often asymmetrical, in that a given stationary stimulus gave unequal responses in corresponding portions of the flanking areas. In a few fields only two regions could be demonstrated, located side by side. Receptive fields could be oriented in a vertical, horizontal or oblique manner. Effective driving of a unit required a stimulus specific in form, size, position and orientation, based on the arrangement of excitatory and inhibitory regions within receptive fields. A spot of light gave greater responses for some directions of movement than for others. Responses were often stronger for one direction of movement than for the opposite; in some units these asymmetries could be interpreted in terms of receptive field arrangements. Of the 45 units studied, 36 were driven from only one eye, 15 from the ipsilateral eye and 21 from the contralateral; the remaining nine could be driven from the two eyes independently. In some binocular units the two eyes were equally effective; in others various degrees of dominance of one eye over the other were seen. Binocularly activated units were driven from roughly homologous regions in the two retinas. For each unit the fields mapped for the two eyes were similar in size, form and orientation, and when stimulated with moving spots, showed similar directional preferences. In a binocular unit excitatory and inhibitory regions of the two receptive fields interacted, and summation and mutual antagonism could be shown just as within a single receptive field.