In the brains of male white rats some structural parameters of granule neurons in the hippocampal dentate gyrus such as cytoarchitectonical arrangement of these cells, structure of dendrites and dendritic spines were analyzed during ontogenetic development as well as changes in these parameters due to modified external conditions (acoustic stimulation, dark rearing, cerebrolysine administration). Transmission electron microscopy as well as cytologic and morphometric methods were employed. Under normal conditions the adult cytoarchitectonical pattern of the dentate gyrus was reached at postnatal day 20 (P 20). At the ultrastructural level the differentiation of the neuronal perikarya during ontogenesis was determined by the size of the nucleus, occurrence of rough endoplasmic reticulum, ribosoms and polysoms, Golgi apparatus and mitochondria between birth and postnatal day 90 (P 0--P 90). Synaptogenesis starts in the perinatal period in form of immature axo-somatic and axo-dendritic contacts. In adults a pronounced structural variability of synapses and the presence of complex spines was found. The average density of spines increased considerably with age in isolated dendritic segments as well as over the whole length of a dendrite. The density was at P--P 2: 0,03; P 5: 0,04; P 8: 0,09; P 10: 0,1; P 15: 0,17; P 20: 0,23; P 30: 0,48; P 180: 0,62 spines per micrometer dendrite. The postnatal development of the neurons under experimental conditions was characterized by significant augmentations of the spine numbers mainly in the early postnatal phase: Controls at P 15 with 0,17, after dark rearing 0,30, after Cerebrolysine 0,32, after acoustic stimulation 0,33 spines per micrometer dendrite. At P 30 only dark rearing resulted in significant augmentation of the spine number (0,63 spines per micrometer dendrite against 0,48 in controls). The data obtained after acoustic stimulation and dark rearing point to a widespread influence of external stimuli on neuronal development not only confined to the corresponding visual or acoustic pathway. The Cerebrolysine results during early postnatal development can be explained by a stimulation of protein synthesis and, hence, of the neurogenesis.