1. In Golgi-Cox-impregnated coronal sections of albino rat brains at 1, 4, 26, 24, 30, 60 and 90 days it is presented the evolution of the spine-less, bare initial zone ("nude zone", NZ) at the proximal apical main dendrites of the layer V pyramidal neurons in the somatosensory and anterior limbie cortex. The quantitative results are analyzed by statistical methods. 2. The NZ is comprehended as a morphological correlate of the endodendritic neuroplasmic flow (Weiss 1944, Globus, Lux and Schuberl 1968, Kreutzberg 1973). The observed changes of the percental frequency and the average length of NZ increases rapidly. 3. The NZ occurs at first in the (12th) 16 postnatal day, thus in a time, when the organs of hearing and the eyes are differentiated completely. Between 16th and 24th day the percental frequency as well as the longitude of NZ increases. During this time the rats will be independent of the mother animals. With the full differentiation of the urogenital tract and especially with the sexual maturity the percentage frequency of NZ increases only at pyramidal cells in the anterior limbie cortex between 24th and 60th day. During 3rd month the NZ is occuring percental more frequently in the anterior limbic cortex than in the somatosensory cortex. At this time the average length of NZ is shorter in the limbic cortex. 4. As to the enriched, vivid movement of the animals and the playing impulse of the young rats the average length of NZ will be extended at pyramidal neurons in the somatosensory cortex during 2nd month, as well as the pattern of spine distribution will be changed along apical dendrites (Schlerhorn, unpublished). During the following (3rd) month the NZ will be shorteded in the somatosensory cortex, obviously caused by new formation of spines at the proximal main dendrites. 5. These newly formed spines in the initial zone of apical dendrites may be inhibitory spines. The inhibitory spines are stained only when using the mercury chromate impregnation according to Golgi-Cox, but not when using the silver chromate methods according to Golgi-Kopsch or Golgi-Bubenaite. The different tingibility of these spines by different Golgi techniques is discussed by Doedens, Nagel and Schierhorn (1974). The pyramidal neurons in the somatosensory cortex possess a longer average length of NZ (Lnz = 7,3[mum]) than the pyramidal cells in the anterior limbic cortex (Lnz = 6.2[mum]). As to NZ the differences between silver and mercury chromate stained pyramidal neurons are greater in the somatosensory cortex than in limbic cortex (see Tab. 7). Therefore we assume that there are in the initial zone of somatosensory pyramidal neurons more inhibitory spines than at the pyramidal neurons in the anterior limbic cortex. 6. The regional differences in the percentual frequency and in the average length of NZ between somatosensory and limbic cortex are new identifying marks of architectonic differentiation of the pyramidal neurons in cortical regions of phylogenetically different ages.