Tropical rainforests in the Amazon Basin and elsewhere are experiencing stronger droughts as El Nino episodes become more frequent and more severe. How will this drying trend affect these species-rich tropical ecosystems? We initiated the world's first large-scale rainfall manipulation experiment in a tropical rainforest in 1998, partially excluding rainfall from a one-hectare forest area in central Amazonia. We discovered that the growth of tree trunks is very sensitive to drought, as is forest flammability. During the next five years, we will determine the level of drought stress that this rainforest can tolerate before tree flowering and fruiting begin to fail, and large trees begin to die. We will then release the forest from experimental drought to document the mechanisms and rates of forest recovery. It is already clear that drought effects on tree growth and tree mortality may contribute substantially to the carbon dioxide concentration of the atmosphere.