The spine morphology of LV--pyramidal neurons in the cingulate cortex was analyzed, using drawings of apical side dendrites without ramifications, by light microscopical analysis of 5300 times magnification in GOLGI-preparations of the brains of 11 and 60 days old control rats as well as of experimental animals reared under starvation conditions from day 1 till day 60 of their postnatal life. The spine density and the relative frequency of three different spine types (thin, mushroom and stubby shaped) was estimated in control and undernourished animals. Undernutrition resulted in a considerable deficit in the spine number of 25% in 11 days old animals, respectively, of 41% in 60 days old animals. Additionally, in 60 days old undernourished rats changes in the relative frequency of the three spine types was found. To the thin type belonged 46% (68% in controls) to the mushroom shaped form 37% (19% in controls) and to the stubby type 16% (13% in controls) of all visible spines. This pattern in 60 days old undernourished rats was very similar to the pattern of 11 days old control animals (thin 43%, mushroom 36%, stubby 22%). The evaluate the differentiation of the dendritic spines during ontogenesis as well as the disturbing influence of undernutrition on these processes, additional data of the spine morphology (neck length and head area) were collected. The spines of the 11 days old animals showed a larger head area (undernourished and controls) than the young adult ones. However, the thin spine type present in 60 days old undernourished rats exhibits morphological features (extremely long necks of about 2,5 micron in comparison to 1,6 micron in controls as well as very large heads) which appeared to be quite similar to the thin and long spine type observed by PURPURA (1975) in human fetal cortex and in cortex of patients with mental retardation. This super long thin spine type is considered to be a less ripe stage of the spine development. The relative high portion of mushroom and stubby shaped spines in undernourished and young animals points to the same assumption or to degenerative changes in the experimental animals.