Many regions of the mammalian brain are characterized by iterated ensembles of nerve cells which can be distinguished anatomically and physiologically. A particularly striking example is the pattern of glomeruli in the olfactory bulbs; other instances are columns and 'blobs' in the visual cortex, barrels and columns in the somatosensory cortex, and striasomes and cell islands in the neostriatum. Understanding the generation of these neuronal ensembles has a bearing on a variety of important neurobiological problems, including the nature of critical periods, the age-dependent response of the nervous system to injury and the manner in which neural information is stored. Analysis of these issues has usually been restricted to studies of the brains of different individuals at various ages. Many questions about the formation of such units, however, can only be answered by observing the same brain repeatedly in a living animal. This strategy would enable a direct assessment of how these units are assembled, whether the initial ensembles persist and whether units are lost or gained as an animal matures. We have succeeded in studying the pattern of glomeruli in the mouse olfactory bulb on two separate occasions during postnatal development. Comparison of the patterns observed at intervals of up to three weeks show that this part of the brain is gradually constructed by the addition of new glomeruli to a persisting population.