Data from the scientific literature and statistical methods were used to estimate the magnitude and significance of the effects of the amount and source of dietary crude protein (CP) supplements on the supply of N fractions passing to the small intestine of lactating dairy cows. The passage of total N and nonammonia N from the rumen was influenced by the source of CP in the control diet and only marginally by the source of ruminally un-degradable protein (RUP) in the dietary treatment. Even though NH3N did not appear to limit growth of the microbes, overall flow of microbial N from the rumen was depressed when RUP partially replaced other sources of CP in the diet. This response tended to be affected by the source of RUP in the dietary treatment. The passage of nonammonia, nonmicrobial N to the duodenum increased when cows consumed diets that contained RUP supplements. The magnitude of this response, however, was distinctly altered by the source of CP in the control diet with which the RUP treatment was compared. In addition, the proportion and source of total CP supplied by RUP in the treatment diet tended to modulate the response. Feeding RUP resulted in a significant increase in the ruminal outflow of total and essential amino acids but the magnitude of this response depended on the source of RUP. Feeding some RUP sources quantitatively improved the delivery of methionine and lysine to the small intestine. Therefore, variability exists in the ruminal outflow of N fractions when different sources of RUP are fed to cows. A portion of this variation is explained by the source of CP in the control diet, the source of RUP in the dietary treatment, the amino acid composition of the dietary CP, and the CP percentage of the diet.