Dendritic spines in the cerebral cortex of newborn and adult guinea pigs were compared quantitatively. An increase in the average diameter of the head by 21% and in the diameter of the stalk by 29% was found. No difference could be measured in the average length of the spines of both age groups. When plotting the individual measurements against each other, a slight correlation could be found only between the size of the spine head and the thickness of the stalk.