Sr isotope ratios in carbonate-rich lacustrine strata provide highly resolved and geographically specific records of past changes in weathering and regional drainage patterns. The Eocene Green River Formation is perhaps the best documented pre-Quaternary lacustrine unit in the world; therefore, these strata are ideally suited for studying the behavior of Sr. The 87Sr/86Sr ratios measured for primary carbonate in lake expansion- contraction cycles of the Laney Member are directly linked to changing lake facies. Four distinct cycles are preserved in a 6.4 m interval of the Arco Washakie Basin No. 1 core, each represented by a vertical succession of transgressive stromatolite facies containing dolomicritic intraclasts, laminated micrite facies deposited during lake highstands, and lake-marginal dolomicrite facies, which represent subsequent lowstands. Initial 87Sr/86Sr ratios range from 0.711 99 to 0.713 31. The least radiogenic isotope compositions are associated with laminated micrite that records increased regional runoff from Phanerozoic marine carbonate and Tertiary volcanic rocks during lacustrine highstands. The most radiogenic isotope compositions are associated with dolomicrite, recording a higher proportion of runoff from basin-bounding Precambrian uplifts and the exposed lake plain during lacustrine lowstand. The observed 87Sr/86Sr variations may also be due in part to changes in differential weathering of the surrounding landscape, where drier climates are associated with decreased differential weathering and, therefore, higher 87Sr/86Sr in runoff.