Stable isotope profiles of fossil freshwater bivalve shells and mammal teeth provide a record of the seasonal δ18O variation in surface waters of the Himalayan foreland over the past 11 m.y. Between 3.1 and 10.7 Ma the δ18O of surface waters approached or exceeded 0‰ standard mean ocean water (SMOW) in the dry season. Since 9.5 Ma the magnitude of seasonal variability in δ18O has remained essentially unchanged. Both observations imply that the Tibetan Plateau had attained sufficient elevation and area prior to 10.7 Ma to support a strong Asian monsoon. These data also imply that the δ18O of wet-season rainfall was significantly more negative (−9.5‰ SMOW) prior to 7.5 Ma than after (−6.5‰ SMOW). If this change is attributable to a lessening of the amount effect in rainfall, this agrees with floral and soil geochemical data that indicate increasing aridity beginning at 7.5 Ma.