The song control nuclei of the zebra finch brain contain more neurones of larger diameter in the male than in the female. This sexual dimorphism is thought to result from differential growth of neurones in the two sexes. Using neurohistological techniques and radioactive tracers, we have studied the development of several forebrain nuclei involved in the control of song and find that the dimorphism arises from neuronal atrophy and death in the female brain as well as from an increase in cell-body size and afferent terminals from other forebrain nuclei in the male. Although the timing of these events varies from nucleus to nucleus, the sequence is essentially similar in all of them except area X. Here we describe the events in one of these nuclei, the robust nucleus of archistriatum (RA), as an example.