Detailed anatomical studies about the distribution of arginine-vasotocin (AVT) gene expressing neurons in the avian brain are so far non-existent. Most of the investigations are limited to discrete brain nuclei in the chicken and concern particular experimental conditions. Since no data on AVT gene expression are up to now available in Japanese quail, we report here the distribution of AVT mRNA containing neurons within the prosencephalon of this bird and a comparison with the same system of the chicken. The AVT mRNA containing perikarya were detected with a specific AVT cDNA probe directed toward the distal 3' glycopeptide encoding part of the chicken AVT gene. The specificity of the probe was tested by Northern and Southern blot analysis. The AVT gene expressing neurons showed the same pattern of distribution in both species; however, a larger number of labelled neurons was observed in the chicken by visual inspection of the sections. AVT gene expressing neurons were distributed within the hypothalamus (in the medial and ventral aspects of the rostral preoptic area, in the lateral forebrain bundle, close to the ependyma of the third ventricle and in several periventricular hypothalamic nuclei) and in some extrahypothalamic regions (around the nucleus rotundus, close to the nucleus commissurae pallii and in a region probably corresponding to the nucleus strial terminalis). AVT gene expressing neurons were observed close to the organum septi laterale of the chicken. Taken together, these results confirm the specificity of the employed probe and its suitability for further studies on the modulation of AVT gene expression in quail.