Vocal behavior in songbirds is sexually dimorphic and many of the telencephalic brain nuclei mediating this behavior are dimorphic and contain receptors for sex steroid hormones. The overall goal of these studies is to understand the functioning of transmitter systems in the sexual dimorphic vocal control circuit and their modulation by hormones. The largest neural sex differences ever described in any vertebrate are the sex differences in the volume of many of the song control brain areas in song birds species such as the zebra finch. The major challenge now facing investigators in the area of neural sex differences is to functionally understand brain differences. With the use of in vitro quantitative receptor autoradiography we are comparing volumetric brain dimorphisms in nuclei that have been defined either based on Nissl staining or based on the density of a neurotransmitter receptor sub-type. To analyze the large amount of data we are generating, an automatic image analysis system is being developed. The approach utilizes parcellation to segment the brain images and to then reconstruct the serial sections into a 3D volume. Volume measurements are then compared for the multiple receptor under investigation.