Androgen modulation of neurotransmitter receptor density was investigated in the nucleus intercollicularis (ICo) of male and female Japanese quail (Coturnix coturnix japonica). ICo appears to play an important role in the neural control of testosterone (T) dependent vocal behaviour. Two receptor types were investigated in this nucleus; muscarinic cholinergic receptors, labelled using [3H]N-methyl scopolamine (NMS) as the ligand, and α2-adrenergic receptors, labelled using [3H]p-amino-clonidine (PAC) as the ligand. Changes in receptor density were assessed using in vitro quantitative autoradiography to ensure a high degree of anatomical specificity in the identification of any steroid effects. Gonadectomy was found to reduce the density of both [3H]PAC bindings sites and [3H]NMS in specific subregions of ICo. Gonadectomized animals treated with T had levels of receptor density similar to intact birds. However, the location within ICo of the effects of T was different for each ligand. [3H]NMS binding was modulated only in a rostral subregion of the nucleus while changes in [3H]PAC receptor density were found in the medial and lateral parts of ICo at a more caudal level. These changes in receptor density parallel changes in crowing frequency that are known to occur in males and females following castration and T treatment. The receptor systems may constitute a part of the neurochemical mechanism regulating vocal behaviour.