We measured water-influx rate during the hot summer in free-ranging adult Arabian oryxes (Oryx leucoryx) in Mahazat as-Sayd, a 2,244-km² protected area in west-central Saudi Arabia. Oryxes obtained 2,294 ml/day of water in their food and from oxidative water, the latter amounting to 14.2% of total water influx. For ungulates living in hot environments, we constructed an allometric curve: log(water-influx rate [liters/day]) = −0.885 0.922·log(body mass [kg]), (r² = 0.77, F = 26.8, P < 0.001, n = 10). The Arabian oryx had the lowest mass-specific water-influx rate (31.5 ml kg^−0.922 day⁻¹), only 32% that of the camel (99.3 ml kg^−0.922 day⁻¹), emphasizing the degree of evolutionary specialization in oryx. Between June and September, oryxes grazed primarily on 3 grasses, Panicum turgidum, Lasiurus scindicus, and Stipagrostis. P. turgidum, taken in largest quantity, had the highest moisture content, 35–45% per g of wet matter. Dry matter intake averaged about 3.1 kg during the summer months; plant intake varied inversely with moisture content.