The main objectives of this study were: (1) to assess the chemical content and the digestibility of Mongolian rangeland forages, (2) to determine in vivo intake of pasture forages from different zones of Mongolian rangelands by ruminants. The year round in vivo intake and digestibility of rangeland forages was determined in desert-steppe, steppe and high mountains in 1991-1995. Animals used were 3 castrated male sheep and 3 castrated Bacterian camels in the desert-steppe, 3 castrated sheep and 3 steers in the steppe, 3 sheep and 3 castrated yaks in the high mountains. Trials were conducted over periods of 21 days in late February (Winter), April (Spring), June (Early summer), July (Summer) and October (Autumn) and, two measurements at each time during 1991-94 were made. During the experimental periods triplicate diet samples from rangeland forages were also obtained. The content and nutritive value of different types of rangelands in Mongolia vary widely according to annual and seasonal climatic conditions, species composition, and growth rate. In late June sugar content of pasture forages averaged 6.8-13.3%, starch 3.1-6.8%, cellulose 24.2-27.5%, lignin 8.5-11.2 %; in August sugar 6.5-10.7%, starch 1.6-4.8%, cellulose 23.9-31.6%, lignin 8.5-12.5%; in October sugar 2.8-4.0%, starch 2.4-4.8%, cellulose 32.3-39.2%; and in April sugar 1.6-4.3%, starch 1.7-4.6%, celloulose 25.1-39.1%, lignin 8.4-14.9%. Diets metabolizable energy per kg dry matter were 7.7-7.9, 6.4-6.7, 5.9-6.6, 6.4-7.2 kJ, respectively. Due to harsh environmental conditions Mongolian rangeland plants tend to have lower lignin content than tropical species. Bacterian camel on desert-steppe pasture consumed 6.92-10.61, 5.55, 5.12 and 10.72 kg/day forage in autumn, winter, spring and summer respectively. Diet digestibilities (DMD) were 52.9, 48.5, 38.9, 55.3%, respectively. Yak on high mountain pasture consumed 3.64, 4.64, 3.95, and 4.37 kg/day forage in autumn, winter, spring, and summer, respectively. DMD diet ranged from 55.1 to 56.5%, respectively. Steer on steppe pasture consumed 9.55, 7.02, 7.89 and 5.10 kg/day in autumn, winter, spring and summer respectively. Diet DMD ranged from 49.5, 39.6, 46.1 and 49.6%, in autumn, winter, spring and summer respectively. From steppe pasture sheep consumed 0.99-1.66 kg/day diet in autumn, 1.10-1.21 kg/day in winter, 0.99-1.42 kg/day in spring, and 0.92-1.22 kg/day in summer time.