Precipitation, local ground water, soil water, atmospheric water vapor, grape leaf and grape berry water just prior to harvest, and grape must during the wine-making process, from the Napa Valley in northern California were collected for stable isotopic analysis. In addition, 27 red wines and 4 white wines produced in the Napa Valley, and 8 red wines produced in Livermore Valley located over 110 km to the southeast, were analyzed for both oxygen and hydrogen isotopic compositions. The isotopic compositions of the grape leaf water fall on a transpiration line with a slope of 2.1, while those of the grape berry water fall on a transpiration line with a slope of 2.8. The stable isotopic compositions of the 27 red wines from the Napa Valley range from -3 to +20? in ΔD and from +4.6 to +10.2? in Δ18O and plot along a line described by ΔD=3.4Δ18O-17.2. The maximum difference in the stable oxygen composition between two wineries 110 km apart is only 1.4?, while the differences between the vintage years within each winery are 4.8 and 5.8? in Δ18O. The stable isotopic composition of the grape water is controlled by transpiration in the weeks prior to harvest, overshadowing all other effects. As a result of the timing of harvest, the red wines are some 4 to 5? more enriched in Δ18O than the white wines. ¿ 1999 American Geophysical Union