Chemical determinations and kinetic ruminal studies of the crude protein (CP) fractions of some tropical forage were made. The non-protein nitrogen, the borate-phosphate insoluble and soluble nitrogen, and neutral and acid detergent insoluble nitrogen were analyzed in the forages tifton-85, elephantgrass, palisade grass, signal grass, perennial soy, molasses grass, corn silage, coast-cross hay, signal grass hay, and in the concentrates soybean meal, cottonseed meal, wheat middling, corn meal and grounded sorghum. The ruminal digestion rate constants of the nitrogen fractions were obtained by in vitro procedure using extracted proteases from ruminal microbiota, and analyzed by means of curve peeling technique. The fractions, expressed as percentage of CP, ranged from 3.68 to 32.83% for fraction A, from 0.58 to 20.07% for fraction B₁, from 5.09 to 75.45% for fraction B₂, from 1.62 to 43.97% for fraction B₃, and 0.93 to 27.73% for fraction C. The degradation rate ranged from 34.77 to 238.24% h^-1; 1.13 to 12.59%. h^-1 and 0.06 to 0.95 %.h^-1 for B₁, B₂ and B₃, respectively . There were important variations on the quantities and in the degradation rates of nitrogen fractions that constituted the studied ruminants feeding and the ruminant feed formulation must consider these variation to adequate the nitrogen and energy in the rumen and intestines.