New formation of stem cells in the subependymal brain layer of a normal adult primate has been documented by light and electron microscopy and through 3H-thymidine (3H-TdR) uptake studies. Serial sections (1.5 micrometer) prepared for autoradiography were examined under the light microscope and then re-embedded for electron microscopy. In individual sections subependymal cells did not appear labeled; however, serial sections revealed that many cells were slightly labeled indicating a low level of 3H-TdR incorporating during DNA synthesis. Dividing subependymal cells were also observed by light and electron microscopy. The poor incorporation of intraperitoneally injected 3H-TdR is due to the blood-brain barrier toward 3H-TdR. Rat experiments with the thymidine analogue, 125I-iododeoxyuridine (IUDR-125I) showed that after intraventricular injection of IUDR-125I about ninety times more radioactivity is incorporated into dividing stem cells of the brain than after an intraperitoneal injection of IUDR-125I. Differential corporation of 3H-TdR seems to depend on the type of stem cells, i. e., whether a stem cell may be precursor for glial or neuronal brain cells. Changes in membrane permeability, intracellular DNA activity as well as difference in locally available "cold" thymidine may be a reason for differential uptake of radioactive thymidine. Differential uptake of 3H-TdR needs consideration when autoradiography is applied on brain cells.