We determined the Δ18O variations in the latewood of tree rings from four silver firs (Abies alba Mill.) for the period 1840--1997 at a mountain site in Switzerland, establishing the longest available tree ring record for Δ18O in central Europe. The isotope ratios were determined on whole wood with a rapid continuous flow pyrolysis technique, thus avoiding cellulose extraction. We found significant correlations with Δ18O tree ring records from the same region, although these involved different materials (cellulose extracted from whole rings rather than latewood) and different species. This indicates that physical factors are more important than biological influences as a determinant of Δ18O in tree rings. The isotope tree ring chronology was highly correlated with the oxygen isotope variations in the June/July precipitation for the period 1972--1992 (r=0.72), and Δ18O in whole wood of tree rings is therefore well suited for the reconstruction of Δ18O in precipitation. We found a slow, quasi-periodic variation of the Δ18O series with a periodicity of ~24 years, which is correlated to variations in the July temperature. This could be caused by fluctuations in the large-scale atmospheric circulation over Europe and the North Atlantic, which may result in a change in source and flow path of atmospheric moisture, affecting the isotope ratio of precipitation in Switzerland. Although a significant correlation with summer temperature was found (p<0.01), the low correlation coefficient (r=0.31) indicates that the Δ18O variations cannot be explained by temperature variations alone. However, even when considering that the factors influencing Δ18O in precipitation are not yet fully understood, our study shows the potential of tree rings to provide long records of Δ18O in precipitation for continental areas, which will improve our understanding of the causes of natural perturbations of the climate system. ¿ 2000 American Geophysical Union