The orientation selectivity of visual cortical cells was tested in two groups of kittens. In one group the animals were reared normally for the first 4-6 weeks of life then kept in darkness. Those in the other group were dark-reared for the first 6 weeks then exposed to light for 6 h and returned to the dark. The properties of the receptive fields of visual cortical cells were examined in these kittens after periods of dark-rearing ranging from 3 days to 12 weeks. In both groups, the proportion of orientation selective cells was found to decrease with time spent in the dark. The critical period for orientation appeared to end at 10-12 weeks of age. Two populations of visual cells were distinguished functionally by their different behaviour during prolonged dark-rearing. Most of the cells which retained their orientation specificity longest during dark-rearing were tuned to horizontal or vertical orientations and more of them were monocular than in normal kittens. These functional characteristics resemble those exhibited by neurons of very young kittens. Changes in specificity observed during loss of selectivity are compared to those observed during early development. We suggest that the extent to which the orientation selectivity of a cell is plastic depends very largely upon the time, during the course of development, at which its selectivity was acquired.