ABSTRACT Macroscale hydrological modelling is currently conducted using Global Climate Models (GCMs) coupled to a range of Soil-Vegetation-Atmosphere Transfer Schemes (SVATs). The most extreme type of simulation involves massive land use change. This paper reports on the results of a tropical deforestation experiment in which the tropical moist forest throughout the Amazon Basin and SE Asia has been replaced by a scrub grassland in a version of the NCAR Community Climate Model (Version 1) which also incorporates a mixed layer ocean and the Biosphere-Atmosphere Transfer Scheme (BATS). In the Amazon we find a smaller temperature increase than did all other previous experiments except Henderson-Sellers and Gornitz (1984); indeed temperatures decrease in some months. On the other hand, we find larger hydrological responses than all earlier experiments including runoff decreases and a larger difference between the changes in evaporation and precipitation which indicate a basin-wide decrease in moisture convergence. Disturbances extend beyond the region of land-surface change causing temperature reductions and precipitation increases to the south of the deforested area in S America. Changes to the surface climate in the deforested area take between 1 and 2 years to become fully established although the root zone soil moisture is still decreasing at the end of a 6-year integration. Besides temperature and precipitation, other fields show statistically significant alterations, especially evaporation and net surface radiation (both decreased). An important question raised by this type of simulation concerns the appropriateness of the microhydrological process models employed in SVATs to the GCMs in which they are currently used.