Incorporating the full geochemical cycles of stable water isotopes (HDO and H2 18O) into an atmospheric general circulation model (GCM) allows an improved understanding of global ΔD and Δ18O distributions and might even allow an analysis of the GCM's hydrological cycle. A detailed sensitivity analysis using the NASA/Goddard Institute for Space Studies (GISS) Model II GCM is presented that examines the nature of isotope modeling. The tests indicate that ΔD and Δ18O values in nonpolar regions are not strongly sensitive to details in the model precipitation parameterizations. This result, while implying that isotope modeling has limited potential use in the calibration of GCM convection schemes, also suggests that certain necessarily arbitrary aspects of these schemes are adequate for many isotopo studies. Deuterium excess, a second-order variable, does show some sensitivity to precipitation parameterization and thus may be more useful for GCM calibration. Due to strong sensitivity over polar regions, GCM isotope modelers must choose carefully the numerical scheme for isotope transport and the formulation of kinetic fractionation processes at snow formation. The GCM results support the assumption that isotope fractionation does not occur during evaporation over continental areas. ¿1991 American Geophysical Union