The organization of the avian telencephalic pallium differs considerably from that of mammalian cortex. While large parts of the mammalian cortex exhibit a six-layered structure, the avian pallium is not laminated as it is in mammals, but is instead nuclear in configuration, with large masses of neurons making up the three regions of the dorsal ventricular ridge (mesopallium, nidopallium, and arcopallium) and the hyperpallium (Wulst). Studies of gene expression and fiber connections have suggested that the primary visual (entopallium) and auditory (field L2) centers in the nidopallium of the avian telencephalon are homologous to layer 4 of extrastriate and auditory neocortices of mammals, respectively. In addition, it has been proposed that the arcopallium contains neurons homologous to layers 5/6 and that the mesopallium may be homologous to superficial neocortical layers, but gene expression evidence for the latter is lacking in adult birds. In the present study using adult chickens and pigeons, I have examined the gene expression of cholecystokinin (CCK) mRNA, a selective marker for layers 2/3 of the mammalian neocortex, by an in situ hybridization method. CCK mRNA was expressed in neurons of the entire mesopallium of the chicken and pigeon but not in any part of the nidopallium. Furthermore, CCK mRNA was expressed in a part of the hyperpallium, i.e. the densocellular part of the hyperpallium (HD) and/or intercalated part of the hyperpallium (HIS). Together with hodological evidence of connections between the mesopallium and the two primary sensory areas, the present results are consistent with the suggestion that the mesopallium of birds is comparable to layers 2/3 of the mammalian neocortex.