We review the current understanding of the Dole effect (the observed difference between the δ18O of atmospheric O2 and that of seawater) and its causes, extend the record of variations in the Dole effect back to 130 kyr before present using data on the δ18O of O2 obtained from studying the Vostok ice core (Sowers et al., 1993), and discuss the significance of temporal variations. The Dole effect reflects oxygen isotope fractionation during photosynthesis, respiration, and hydrologic processes (evaporation, precipitation, and evapotranspiration). Our best prediction of the present-day Dole effect, +20.8‰, is considerably lower than the observed value, +23.5‰, and we discuss possible causes of this discrepancy. During the past 130 kyr, the Dole effect has been 0.05‰ lower than the present value, on average. The standard deviation of the Dole effect from the mean has been only ±0.2‰, and the Dole effect is nearly unchanged between glacial maxima and interglacial periods. The small variability in the Dole effect suggests that relative rates of primary production in the land and marine realms have been relatively constant. Most periodic variability in the Dole effect is in the precession band, suggesting that changes in this global biogeochemical term reflects variations in low-latitude land hydrology and productivity or possibly variability in low-latitude oceanic productivity.