Abstract: Gynandromorphs, rare in vertebrates, exhibit distinct sex-determining gene expression on each side of the body despite sharing a uniform hormonal environment. This provides a unique opportunity to investigate the respective roles of genes and hormones in sex determination. We accidently obtained a gynandromorphic Zebra Finch with a male-female chimeric appearance but only with an ovary-like gonad. Its plasma estradiol was significantly higher than that of age-matched females, and its sexual partner preference was also feminine. Although it did not sing like males, its calls showed masculinization. In the brain on one side of the body with male plumage, the area of song motor nucleus, the robust nucleus of the arcopallium (RA), and the excitatory synaptic transmission of RA projection neurons showed masculinization. Transcriptome analysis revealed that genes related to cholinergic neuron function were significantly upregulated in the masculinized side of brain. Moreover, there were extensive and consistent expression differences of neuroactive substance receptor genes in both sides of body, indicating that cell-autonomous determination plays a key role in sex dimorphism of neuromodulation.