Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder that affects primarily the striatum and cerebral cortex. A search for the factors that increase the vulnerability of striatal neurons will lead to a better understanding of the pathological cascades of this disease. A current hypothesis for neurodegeneration of striatal medium‐sized spiny neurons in HD is an alteration in N‐methyl‐D‐aspartate (NMDA) receptor function. In the present study we examined electrophysiological properties of NMDA receptors in the R6/2 transgenic mouse model. These animals express exon 1 of the human HD gene and present an overt behavioral phenotype at about 5 weeks of age. Whole‐cell voltage clamp recordings from acutely dissociated striatal neurons were obtained from three different age groups of transgenic mice (15, 21, and 40 days old) and their littermate controls (WT). In transgenic animals, two groups of neurons were found with respect to NMDA and Mg²⁺ sensitivity. One group of R6/2 cells displayed responses similar to those of WT, whereas the other showed increased responses to NMDA and decreased Mg²⁺ sensitivity. These cells were encountered in all age groups. The abnormal sensitivity to NMDA and Mg²⁺ indicates that NMDA receptor alterations occur very early in development and suggest the presence of constitutively abnormal NMDA receptors. These alterations may contribute to an enhancement of NMDA responses at hyperpolarized membrane potentials that may be a key factor in striatal neuronal dysfunction. © 2005 Wiley‐Liss, Inc.