Song control regions in passerine birds are known to be sexually dimorphic in the adult brain in species like zebra finches in which most males sing whereas females do not. As a first step toward the analysis of the establishment of anatomical sex differences, volumetric changes of Nissl-defined song control regions in the forebrain of the zebra finch have been quantified in both sexes at 10-day intervals starting at day 10 posthatching. In males, the Nissl-defined volume of the high vocal center, the robust nucleus of the archistriatum, and area X of the lobus parolfactorius increased with age, reaching the adult value at 60, 50, and at 40 days posthatching, respectively. The lateral magnocellular nucleus of the anterior neostriatum increased in volume until 20 days and then gradually decreased to reach the adult value at about 40 days. Whereas area X is absent in females, the high vocal center, the robust nucleus of the archistriatum, and the lateral magnocellular nucleus of the anterior neostriatum were detectable throughout development and in adulthood. In contrast to the males, volumes of the high vocal center and of the robust nucleus of the archistriatum decreased in females between 10 and 40 days posthatching (58% and 61%, respectively), when adult values were reached. Contrary to the development of these two nuclei in females, the volumetric development of the lateral magnocellular nucleus of the anterior neostriatum was very similar in both sexes. Females began with a smaller lateral magnocellular nucleus of the anterior neostriatum at 10 days posthatch, which led to a sexual dimorphism in juvenile stages, but there was no sexual dimorphism of volume size in adult brains.