1. 284 single cortical neurones were studied in area seventeen of twenty-five normal kittens and of fifteen kittens, binocularly deprived, whose first visual experience had been delayed until the experiment by bilateral lid-suture. Both normal and binocularly deprived kittens ranged in age from 1 to 6 weeks.2. The optimal, binocularly presented, visual stimulus and receptive fields were determined for each neurone by varying target configuration, speed and direction of movement and the prism-induced alignment of both eyes. Repetitive, controlled stimulation in eighty-four cases allowed quantitative estimates to be made of the response selectivity for the target configuration (spot vs. line), the direction of target motion and the prism-induced disparity between the retinal images of the binocular target.3. Before the fourth post-natal week neurones from both normal and binocularly deprived cortex showed similar properties: selectivity for direction of target motion was present in both preparations but both lacked binocular specificity and dependence on target configuration.4. After the fourth week, normal kittens had increasing numbers of neurones with selective responses which were dependent upon target configuration and the degree of binocular misalignment. The proportion of selective neurones approached the adult value after the fifth week.5. The cortex of binocularly deprived kittens failed to show an increase of selectivity with age, and of 150 neurones, sixty-two were visually unresponsive, two showed selectivity which was dependent upon target configuration and none showed selectivity for prism-induced retinal disparity.6. The data are not consistent with the hypothesis that the highly specific response properties of visual cortical neurones can develop without appropriate visual experience. Innate mechanisms appear to be sufficient for the development of the excitatory connexions producing motion sensitivity and receptive field location on both retinas, but patterned visual experience is necessary for the ;fine-tuning' which vetoes responses to stimuli with non-optimal configuration or binocular disparity.