This study examined the development of individual axon terminal fields in n. laminaris (NL) of the chicken brainstem. In their mature form axons from the nucleus magnocellularis (NM), second-order auditory neurons in the chicken brainstem, project bilaterally onto the NL. Axons from the ipsilateral and contralateral NM neurons form spatially segregated, elongated arbors in the dorsal and ventral neuropil of NL, respectively. The long axes of these arbors correspond to physiologically defined isofrequency bands. To assess the development of this stereotyped arborization pattern, 6-17-day embryonic chicken brain stems were maintained in vitro while injecting horseradish peroxidase into small groups of axons. Three-dimensional reconstructions were made from serial sections and projected onto a cartesian plane for quantitative analyses. At embryonic day 6 (E6), the ventral axons already course beneath the recently migrated NL neurons. The arrival of the dorsal NM axon branches is delayed and their paths are indirect. They first loop dorsally into the the ventricular layer, where they seem to make specific connections with migrating NL neurons and use these as guides to their appropriate positions in the NL. During the period from E9 to E17 the dorsal and ventral terminal fields become similar, each adopting properties of the other's initial pattern. The dorsal terminal fields extend to form bands similar to the early ventral terminal fields, while the ventral terminal fields narrow and appear to shift position in order to achieve the tonotopic specificity characteristic of the early dorsal terminal fields. The results show that a complex, mature pattern of neuronal connections can be formed during development by the combination and reorganization of two simple patterns--each shaped, in turn, by its respective axonal trajectory.