The cycles of the water isotopic species (HDO and H2 18O) have been incorporated into the NASA Goddard Institute for Space Studies (GISS) atmospheric general circulation model (GCM). The results of a 3-year simulation for present-day conditions are discussed, with special emphasis on the comparison between predicted and observed isotopic distributions (well documented mainly through the International Atomic Energy Agency/World Meteorological Organization network) for both the seasonal and annual time scales. The observed seasonal cycles are generally well simulated. For the annual scale the observed linear relationship between Δ18O and the surface temperature at middle and high latitudes, as well as the absence of any correlation between these fields in tropical and equatorial regions, are properly obeyed by the GCM simulation. In the tropical and equatorial regions the Δ18O patterns for both observations and the GCM are influenced by the amount of rainfall. There is excellent agreement between the simulated and observed ΔD--Δ18O relationship throughout the world. ¿ American Geophysical Union 1987