The postnatal development of layer VI pyramidal neurons in the cat's striate cortex has been studied by means of intracellular injections of Lucifer yellow in aldehyde-fixed tissue (LYF technique). It is shown that the LYF technique gives results qualitatively and quantitatively similar to results obtained with other techniques (Golgi, marker-injections in viable tissue). Quantitative analysis demonstrated significant increases in soma diameter, number and length of basal dendrites, length of second order apical dendrites and, in particular, in number of spines/unit dendritic length, during the first postnatal month. Maturation of the basal dendritic tree and increase in number of spines continue in the second postnatal month. At later postnatal times soma diameter and number of spines decrease by about 20%. Dendritic varicosities are most frequent during the first postnatal week, and decrease in number steadily from thereon. The late maturation of layer VI pyramidal neurons suggests that these cells might be affected by early peripheral lesions and/or sensory deprivation to which the striate cortex of the cat has been shown to be most susceptible around the end of the first postnatal month.