In 6 month old male rats the structure of dendrites and the distribution of spines on the apical dendrites of granular cells of the dentate gyrus were investigated by light microscopy and statistical methods. 2. The number of dendrites of the first, second and third order of granular cells increases in this sequence in a ratio of 1:2:3; the total length of the dendrites increases correspondently in a ratio of 1:4:5. 3. The mean number of origin points of dendritic branches is 10, the mean number of free dendritic endings is 12. 4. The number of spines per a 25 mum dendritic segment near to the pericaryon (dendritic segment A), in the middle of the dendrite (dendritic segment B) and in the peripheral dendritic part (dendritic segment C) as well as the distribution of spines in the whole apical dendrite was evaluated. The total mean of spines of granular cell apical dendrites of the dentate gyrus (superior respectively inferior) is 12 respectively 10 for the dendritic segment A, 18 respectively 17 for the dendritic segment B and 17 respectively 15 for the dendritic segment C calculated for a dendritic length of 25 mum. 5. The spine density in each case depends upon the distance pericaryondendritic segment and is in close relation to the adjacent layers with their specific afferents. 6. The averaged total number of spines per 1 mum of dendritic length is 0,62 spines/mum for the dentate gyrus (superior) and 0,57 spines/mum for the dentate gyrus (inferior). 7. The granular cells of the dentate gyrus (superior) have a mean dendritic length of a total of 357 mum with a total of 226 visible spines; the granular cells of the dentate gyrus (inferior) have a mean dendritic length of a total of 450 mum with a total of 258 visible spines.
