The tropical hydrologic cycle affects atmospheric trace gases and global climate change, and thus records of hydrologic change encompassing a variety of time scales from the low latitudes are important in paleoclimatology. Isotopic analysis of calcite from Lake Junin, Peru, provides a record of hydrologic variability that spans the last glacial-interglacial transition in the southern tropics. The record reveals a 6‰ enrichment in δ18Ocalcite during the late glacial followed by a gradual depletion during the Holocene, which can be interpreted as a decrease followed by a long-term increase in effective moisture. Close agreement between δ18Ocalcite and rainy season insolation indicates that long-term changes in tropical hydrology were linked to orbital variations. Furthermore, hydrologic change was out of phase in the northern and southern tropics over this time period.