In ontogenesis of rats, similar to many other animals, in the development of the spontaneous motor activity two qualitatively different stages are observed. The first one is characterized by periodic, mainly sensor-independent generalized motor excitation which at the second stage is substituted by stereotypic specialized motor programs (locomotion, grooming). Both stages exhibit similar age dynamics: the increase in the activity, its maximum and subsequent inhibition with a shift of the corresponding phases for about two weeks. This dynamics is mainly associated with heterochronous maturation of excitatory and inhibitory systems of regulation of the nervous activity. Change in the stages depends on changes of functional role of motor activity in ontogenesis from mainly morphogenetic one (and promotion of a necessary level of vegetative functions) to directional behavior. Inhibition at the first stage results in deep functional rearrangement in the central nervous system serving as a background for the onset of specialized behavioural activity. The development of the latter is also excessive, the subsequent inhibition being less deep and reversible.