Groundwater from the Ojo Alamo and Nacimiento aquifers in the central San Juan Basin. New Mexico, has yielded 14C ages ranging from modern to 35,000 yr B.P. The Pleistocene-age samples are characterized by a stable isotope content about 25‰ lighter in D and 3‰ lighter in 18O than modern precipitation and groundwater. We attribute this difference to a colder mean annual temperature and perhaps increased winter precipitation. Consideration of various factors controlling the stable isotope composition of the groundwater allows estimation of a 5° to 7°C temperature decrease during the late Wisconsin, accompanied by increased effective precipitation. A similar estimate of the temperature change is obtained from noble-gas paleothermometry. These data support a model of moderately cooler late Pleistocene climate in the American Southwest characterized by summers with less precipitation than today, but wetter winters.