Normal kittens showed a gradual increase from 17.6% to 25.5% of BLS with aging from the fifth week to the ninth. Young kittens showed significantly less BLS than do adult ones (p < 0.1), There was significant difference of BLS between exotropic and normal kittens or alternating and non-alternating strabismic ones. However, there was a significant diftrence between esotropic and normal ones (p < 0.1). These results suggested that the BLS can be used as a index for the extent of total field from both eyes.