The study was performed on mice, rats, and cats. The techniques used were those of conditional reflexes and evoked potentials. The results obtained give proof of the existence of hemisphere specialization of the brain in animals, and some of its properties are described. Motor control is shown to be mainly connected with the left hemisphere. Spatial analysis is mainly lateralized in the right hemisphere, and time analysis in the left one. It has been found that differentiation of absolute characteristics is mainly determined by the right hemisphere, and of relative ones by the left hemisphere. In males the brain has been shown to be more asymmetrical than in females. Proof is given of genetic determination of interhemispheric asymmetry magnitude. It is suggested that interhemispheric asymmetry arose not under the influence of speech and right-handedness but is determined by some more fundamental factors.