Two KIR (K+ Inwardly Rectifying) channel genes have been identified on chromosome 21, in a region associated with important phenotypic features of trisomy 21, including mental retardation: KIR3.2 (GIRK2) and KIR4.2. We analysed the expression of these channel genes in developing human and mouse brains to determine the possible role of the corresponding channels in brain development and function. KIR3.2, which has been extensively studied in the mouse, was found to be expressed in the human cerebellum during development. The KIR4.2 channel is expressed later in development in both mice and humans. We compared the expression of these channels in terms of RNA and protein levels and discussed the potential synergy and consequences of the overexpression of these channels in Down's syndrome brain development.