We have determined the age dependence of the characteristics of the cytoplasmic phase separation of lenses from normal and galactosemic young rats. In the normal lens, the temperature at which the phase separation occurs decreases monotonically with age. In the lenses of rats fed with a high galactose diet, the phase separation temperature becomes increasingly higher with the development of galactosemia. When the phase separation temperature becomes higher than the ocular temperature, the nuclear opacity appears in vivo. The opacity is the result of light scattering by spatial fluctuations of the refractive index formed by interspersed regions of two separated phases in the fiber cell cytoplasm. This shows that the nuclear opacity that develops in the lens of galactosemic rats is the manifestation of phase separation of the lens fiber cytoplasm.