We have explored basic rules guiding the early development of topographically organized projections, employing the rat corticopontine projection as a model system. Using anterograde in vivo tracing with 1,1',dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), we studied the distribution of labelled fibers in the pontine nuclei in relation to cortical site of origin during the first postnatal week. Labelled corticopontine fibers enter the pontine nuclei in distinct, sharply defined zones. The putative terminal fibers typically occupy lamella-like subspaces. Related to changes in cortical site of origin, we describe mediolateral, internal to external, and caudorostral distribution gradients in the pontine nuclei. Fibers originating in the anterolateral cortex occupy an internal central core, while implantations at increasing distance from the anterolateral cortex produce 1) more externally located lamellae, and 2) a caudal to rostral shift in fiber location. Previous investigations have shown that pontocerebellar neurons migrate into the ventral pons in a temporal sequence (Altman and Bayer [1987] J. Comp. Neurol. 257:529). The earliest arriving neurons occupy the central core and later arriving neurons settle in more externally and rostrally located subspaces. We hypothesize that the earliest arriving corticopontine fibers grow into the then only available zone of pontocerebellar neurons (central core), attracted by a diffusible chemotropic cue. Later arriving fibers grow into correspondingly later and more externally and rostrally located contingents of pontocerebellar neurons. Thus, we propose that the topographical organization in the early postnatal corticopontine projection is determined by simple temporal and spatial gradients operative within source (cerebral cortex) and target region (pontine nuclei).