The gene expression of five NMDA receptor channel subunits, the ɛ1 ɛ2, epsiv;3, ɛ4 and ζ1 subunits, was examined in cerebellar Purkinje cells of the staggerer mouse at postnatal day 21. In the midline region of the staggerer cerebellum, signals for the ɛ1, ɛ4, and ζ1 subunit mRNAs were distributed in Purkinje cells, which have a large cell body aligned in a monolayer between the granular and molecular layers. In addition to the midline region, labelled neurons in the intermediate cerebellar region were, though at lower levels, aligned almost in a monolayer between the granular and molecular layers. In the hemisphere, most labelled neurons occurred in various locations in the granular layer and the cerebellar medulla. These regions, populated with Purkinje cells expressing the ɛl, ɛ4, and ζ1 subunit mRNAs, were separated from each other by narrow gap regions that contained neurons without any detectable NMDA receptor channel subunit mRNAs. These results suggest that there is discrete mediolateral heterogeneity in staggerer Purkinje cell populations, in terms of expression properties of the NMDA receptor channel subunits. When compared with wild-type Purkinje cells that express the ζ1 subunit alone, additional expression of the E subunits presumably explains the persistence of NMDA responses in adult staggerer Purkinje cells (Dupont et al Neuroscience, 12, 613-619, 1984).