Approximately 80% of the activities of cortical cholinergic marker enzymes are restricted to afferent fibres and terminals of the magnocellular basal nucleus of Meynert (NbmM) (Wenk et al. 1980). In the evolution this nucleus strongly follows a phylogenetic gradation (Brockhaus 1942) which is positively correlated to the cerebralization, i.e. the neocortical development which an animal species has reached (Gorry 1963). Moreover, in contrast to the brain stem, in vertebrate brains of different phylogenetic levels only the telencephalon showed a clear progressive representation of cholinergic markers (Wächtler 1980, 1981). However, the increased cholinergic activity of the telencephalon in higher forms can not be ascribed to a special hemispheric component (Wächtler 1982). Using the quantitative-histochemical evaluation of the cholinergic marker enzyme acetylcholinesterase (AChE) in the epithalamus (nucleus medialis habenulae) and in different cortical areas of rats during postnatal ontogenesis it is shown that (i) the cholinergic differentation in the thalamic structure seems to be already finished immediately after birth and (ii) the cholinergic synaptogenesis in the cerebral cortex does not start until birth, but takes place postnatally in phylogenetic graduated cortices in temporal echeloned steps (paleo- greater than archi- greater than neocortex). These ontegenetic results correspond to the phylogenetic trends and reflect - in accordance with the biogenetic basic rule - the phylogenetic gradation in the development of the cholinergic basal nucleus of Meynert and its cortical projections shortened in ontogenesis.