Interhemispheric asymmetry of motor activity as well as of emotional and investigating behaviour of male and female rats was studied in the open field. With this purpose during 1 hour the number of square intersections, raising to hinds and peeping into the holes as well as the number of defecations and urination was recorded in intact rats and rats with unilateral decortication. Inactivation of the hemispheres was carried out by means of potassium spreading depression. In male rats, in the first minutes of the exposition and in the second part of the session the left hemisphere dominated during motor and investigating activities. In female rats in the second part of the experiments the right hemisphere dominated. For emotional reactivity the left hemisphere dominated in male rats.