Incorporating stable isotope physics in a general circulation model (GCM) provides a promising means to study isotopic variability in precipitation, including the processes that cause isotopic variability in paleoclimatic archives such as ice cores. This paper describes the implementation and validation of stable isotope tracers in the GENESIS 2.0 GCM. The model reproduces the main features of present-day isotopic fields and the characteristic large-scale isotope-climate relationships. Global δ18O-δD, temperature-δ18O, and precipitation-δ18O relationships are well simulated, and the modeled regional patterns associated with continental vapor recycling over Europe and vertical gradients agree well with observations. In GENESIS a more sophisticated parameterization of interactions between precipitation and atmospheric vapor contributes to a better simulation of isotopic variations in dry climates. The standard model underestimates the global mean deuterium excess (δD-8δ18O) in precipitation, although a heuristic sensitivity test suggests this may be remedied by accounting for nonneutral stratification in isotopic evaporative fractionation over ocean. Errors in simulated isotopic fields are analyzed to determine whether they are caused by local climatic biases in the GCM or by inaccurate parameterizations of isotope physics. Using the results of sensitivity experiments and comparisons with other isotopic GCM results, we identify key isotopic and climatic processes at the origin of the main errors and suggest additional studies to improve isotope simulations.