Spontaneous and evoked unit activity was investigated in the visual cortex of mice with the "ocular retardation" (or/or) mutation, in which the action of the gene is manifested phenotypically by defective development of the optic nerve, with the consequent total blindness of the animals. Control experiments were carried out on inbred C57Br mice. A raised level of spontaneous activity was found in the neurons of the mutant animals and also differences in the distribution of the cells on the basis of the types of their spontaneous activity: A regular type of activity was found 2.5 times more often in the "or/or" mice than in the control group, whereas the proportion of cells with a volley type of discharge was 2.7 times smaller. In addition, visual cortical neurons of the "or/or" mice were more able to respond to acoustic stimulation, when 78% of the responses were tonic in character. Of the unit responses to electrical stimulation of the skin 70% also were tonic, and most were responses of excitation. In 42% of visual cortical neurons of the mutants convergence of heteromodal afferent influences was observed. The functional features described are evidently phenotypical manifestations of the action of the mutant gene on cortical neurons.