The crude protein content, water content and in vitro digestibility of grasses in the diet of the Arabian oryx Oryx leucoryx were investigated. In the main food species, Stipagrostis spp., levels of crude protein, water content and digestibility were generally highest in the flower component while in other species, leaf was the best quality food. Crude protein content, water content and digestibility were highly correlated and all declined exponentially following rainfall. Levels remained comparatively high, even during extended drought, largely because fog and dew-fall enabled some plants to continue to produce green leaf material. The crude protein content of rumen material from oryx that died during drought was negatively correlated with time since rainfall. Estimates of dietary crude protein derived from the protein content of rumen material were comparable to those predicted for available grass forage. The results are discussed in the context of the food and water requirements of oryx. It is suggested that during drought some grasses continue to be a source of adequate crude protein and water and of sufficient digestibility but that levels are close to minimum maintenance requirements. This and the decreasing biomass of available forage of sufficient quality lead to condition loss and death, particularly of the most vulnerable animals. Crude protein may become limiting sooner than does water.