A study was conducted to evaluate the prediction accuracy of the National Research Council (2001) model for metabolizable protein (MP) allowable for milk production. Thirty multiparous Holstein cows were used in a completely randomized design with a 2 × 2 factorial arrangement of treatments. Dietary treatments comprised 2 levels of rumen degradable protein (RDP; 10 and 8%) and 2 levels of rumen undegradable protein (RUP; 8 and 6%) as follows: 10RDP:8RUP, 8RDP:8RUP, 10RDP:6RUP, and 8RDP:6RUP. The 10RDP:8RUP diet was fed from d 1 to 21 followed by respective treatments from d 22 to 42. Cows were exposed to the warm climates of July and August in Tennessee without supplemental cooling. Least squares means of dry matter intake, milk, and body weight from each treatment were input into the model, and tabular crude protein and fiber contents were adjusted to reflect chemically derived values. Treatments did not affect feed intake. The RDP treatment did not affect milk yield but, at 6% RUP, milk yield declined compared with the 10RDP:8RUP diet (RDP x RUP interaction; P < 0.01). The NRC model overpredicted a decline in milk yield (1.9 kg) in response to lowering RDP at 8% RUP (10RDP:8RUP vs. 8RDP:8RUP). At 6% RUP, the model predicted a decline in milk yield (2.0 kg) in response to lowering RDP (10RDP:6RUP vs. 8RDP:6RUP); however, milk yield increased by 2.3 kg. At 10% RDP, the model overpredicted a decline in milk yield (5.1 kg) in response to lowering RUP (10RDP:8RUP vs. 10RDP:6RUP). At 8% RDP, the model overpredicted a decline in milk yield (7.5 kg) in response to lowering RUP (8RDP:8RUP vs. 8RDP:6RUP). Reduction of dietary RDP decreased predicted RDP supply and increased RUP requirements. Reduction of dietary RUP decreased predicted RUP supply but did not affect RUP requirements. In summary, the NRC model underestimated RDP supply and overestimated RUP requirements in response to low dietary RDP. The model underestimated the RUP supply in response to low dietary RUP. An improvement in predictions of MP allowable for milk production should increase the accuracy of the NRC model.