The Cornell Net Carbohydrate and Protein System (CNCPS) is primarily a nutrient supply model, since it does not model the utilisation of the absorbed nutrients, other than by using the NRC (1988) net energy and NRC (1985) available protein systems to calculate the animal's energy, protein and amino acid (AA) requirements. The model incorporates details of the metabolism in the rumen not found in other published models of energy and protein requirements, particularly rates of carbohydrate degradation, predicted rumen pH and rumen nitrogen and peptide balance. The model does not predict volatile fatty acid (VFA) proportions or methane production. Microbial growth is assumed to be dependent upon the rate of carbohydrate degradation, whereas other mathematical models of the rumen relate microbial growth to the concentration of nutrients in the rumen, and also require estimates of rumen volume and microbial mass. The microbial yield of structural carbohydrate (SC) bacteria is not limited or altered by the rumen ammonia supply, with the result that stoichiometrically unsound amounts of microbial protein and negative rumen ammonia levels can be predicted. Only estimated peptide supply modifies the growth of non structural carbohydrate (NSC) bacteria. Solid outflow rates adopted are low at higher levels of feeding compared to ARC (1984) and AFRC (1992), with the result that the proportion of OM digested in the rumen is predicted to be higher (c. 0.75) than the mean value 0.65 adopted by ARC (1980). The effects of liquid outflow rate upon the outflow of NPN, AA and soluble proteins, pectins, sugars, organic acids and VFA are ignored. Only peptides are affected by the liquid outflow rate in the model. The degradation rates for carbohydrate fractions A and B1 proposed are very high, exceeding the possible rate of microbial growth so that microbial synthesis does not respond to considerable variations in these high rates. The adopted maximum microbial yields of SC and NSC rumen bacteria are lowered by 20% to allow for the effects of protozoal predation, which has the effect of compensating for the high predicted microbial yields adopted, but the protozoa are not accounted as contributing to the microbial AAN output. Starch disappearance in the rumen is not corrected for protozoal ingestion and its re-appearance in the intestine, nor is there is any accounting for the protozoal contribution to fat uptake in the rumen. Predicted TDN values of forages are therefore sensitive mainly to the rate of cell wall degradation selected. Only one dietary parameter, effective neutral detergent fibre (eNDF%) defines or modifies maximum microbial yield, microbial maintenance, realised microbial yield, and rates of SC and NSC degradation. The consequence is that both energy and protein supply to the cow are affected by the parameter eNDF when values fall below 24.5% in diet DM.