Unique constraints on seasonal and spatial variations in recharge to the Pleistocene limestone aquifer of Barbados are obtained from the analysis of oxygen isotopic compositions of groundwater and rainwater. Conventional methods of estimating recharge are based on groundwater chloride variations, coastal groundwater discharge, and potential evapotranspiration. These methods typically yield estimates of recharge for Barbados that range from 9% to 20% of average annual rainfall, with significant uncertainties that arise from poorly constrained model input parameters. Owing to the low relief and tropical climate of Barbados, variations in rainwater and groundwater Δ18O values are primarily influenced by the amount of rainfall, with negligible temperature or altitude effects. Composite monthly rainwater Δ18O values are inversely related to rainfall, while groundwater Δ18O values show little seasonal variability. Rainwater Δ18O values are equivalent to groundwater values only at the peak of the wet season. By using mass balance, the difference between groundwater and weighted-mean rainwater Δ18O values gives recharge values. These values are in general agreement with estimates by conventional methods (10--20%) and provide unique additional information including the following: (1) Recharge is restricted to the wettest 1--3 months of the year, and (2) there is less recharge at higher elevations. The effective shift in Δ18O values between contemporaneous rainwater and groundwater via recharge is a useful tool for estimating temporal and spatial variability in recharge and must be considered in paleoclimatic studies where climate inferences are based on groundwater Δ18O values preserved in the geologic record. ¿ 2000 American Geophysical Union