Genes from the Down syndrome (DS) critical region of human chromosome 21, which contribute to the pathology of DS, are also found on mouse chromosome 16. Several animal models of DS with triplication of genes from the DS critical region have been generated, including mouse trisomy 16 (Ts16) and a partial trisomic mouse, Ts65Dn. Using computer-assisted imaging of fura-2 fluorescence, we found an elevation of intracellular cytoplasmic calcium in cortical astrocytes from neonatal Ts65Dn mouse brain, similar to that observed previously in embryonic Ts16 astrocytes. Furthermore, astrocytes from both Ts65Dn and Ts16 cortex fail to respond to the anti-proliferative actions of glutamate. These results suggest that defective regulation of cell proliferation and cellular calcium can result from triplication of DS critical region genes. © 2003 Wiley-Liss, Inc.