Nd isotopic data were collected on Precambrian crystalline rocks exposed in northern, eastern, and southern Mexico, as well as from lower crustal xenoliths from central Mexico, in order to constrain the age and character of the Mexican basement. The data indicate that basement belonging to the Grenville (1.0 Ga) tectonothermal event extends from Los Filtros, in Chihuahua, northern Mexico, to Oaxaca, in southern Mexico. These rocks all have average Nd crustal residence times (TDM ages) in the range 1.60 to 1.35 Ga. We infer that this results from mixing average 1.9 Ga or older recycled continental crust with 70% to 90% newly derived mantle-crustal material during the Grenville orogeny. To the west of the Precambrian, the basement contains large amounts of Phanerozoic (probably Paleozoic) crust, identified from lower crustal xenoliths with TDM ages less than 1.0 Ga. The crust represented by these xenoliths may have been emplaced as suspect terranes in Mesozoic Cordilleran events. Alternatively, the apparent Paleozoic crust that underlies parts of central Mexico may connect to the Paleozoic metamorphic Acatlan complex in southern Mexico, and together they would constitute a continuation of the Appalachian-Caledonian orogenic belt through Mexico. Our data do not preclude either of these two models.