The pattern of dendritic development of layer V pyramidal cells in the neocortex of the rat was studied using a variety of quantitative techniques in an attempt to determine what rules govern dendritic differentiation. Animals were sacrificed on postnatal days (P) 1, 3, 5, 7, 10, 15, 20, 25, 30 and 60, their brains impregnated with the rapid Golgi technique, and cells from the sensorimotor cortex examined for maximal apical and basilar dendritic field, number of dendritic branches at 20 micron intervals from the cell body, number of apical and basilar branch types (branching order), length of dendritic branch segments, and dendritic spine density. Primary dendrites are formed early in development, with no new ones formed after P7-10. Once a dendritic segment has bifurcated, all further development appears to occur at the tip, i.e. the trunk does not seem to undergo additional elongation, and new branches do not appear to form from the trunk. There is a plateau in dendritic differentiation close to the cell body after approximately P20; however, there is a continued increase in the length of terminal dendritic branches in the distal portions of the dendritic field into adulthood. During early development, dendrites bifurcate on reaching approximately 20-30 microns; however, during adulthood additional length is added to terminal dendrites without branching. Dendritic spines increase dramatically early in development, and then decline on proximal dendrites but continue to increase on terminal branches into adulthood. These results suggest that the terminal portion of the dendritic field remains plastic into adulthood, and that during development several general rules govern the pattern of dendritic differentiation.