The early postnatal development of interstitial cells (IC) in the white matter of the temporal cortex in kittens was studied. Counts in Nissl-stained preparations show that the number of IC diminishes by about 60% during the second postnatal week. In Golgi preparations, IC are bipolar or bitufted with long, beaded dendrites coursing in the white matter toward the ventricular surface. Ascending, shorter dendrites are thinner, often branch in a short bush, and possess long spines resembling filopodia. The majority of their axons descend in the white matter, emitting numerous recurrent collaterals that become ascending fibers reaching various cortical layers. Most IC resemble inverted pyramidal cells. They appear well developed at the time of birth and continue to develop elaborate axonal complexes in the white matter of older animals. Electron microscopic observations of degenerating IC were detected in all cases studied and their presence was related to the existence of cell death responsible for elimination of a fraction of IC. They were recognized by their dark aspect and by dilations of the endoplasmic reticulum. Synapses contacting degenerating profiles were also observed. It is concluded that IC belong to the population of early generated subplate cells which may have a transient function involved in certain morphogenetic events during the development of the cortical plate. Some persist in the adult where they can be recognized as IC of the white matter.