There is continuous culture (Calsamiglia et al., 2008) and in situ (Devant et al., 2001) evidence that nonammonia nonmicrobial N (NANMN) flow from the rumen is increased under high concentrate-low pH conditions commonly found in high producing dairy cows. The objective of this study was to evaluate the effect of dietary conditions, including ruminal pH, on the ruminal outflow of N fractions in lactating dairy cows. Peer-reviewed published studies (n = 45 papers; n = 176 data points) designed to investigate flow of N fractions to the small intestine in dairy cows were used. Rumen pH, organic matter truly digested in the rumen (OMTDR), efficiency of microbial protein synthesis (EMPS), and intakes of RUP (RUPI), N (NI), OM (OMI), DM (DMI), and concentrate (CONCI) were the independent variables. Microbial N (MN) NANMN, and nonammonia N (NAN) were the dependent variables in the analysis. Study was included as random effect. The relationship among variables was subjected to multivariate regression analysis by MIXED procedure of SAS. The contribution of each independent variable to the changes observed in each dependent variable was assessed by calculating semipartial correlations (pR²). The RUPI (P < 0.0001; pR² = 0.47), OMTDR (P < 0.0001; pR² = 0.10), EMPS (P = 0.003; pR² = 0.02) and CONCI (P = 0.05; pR² = 0.01) explained 60% of the variation in NANMN. The contribution of rumen pH to the changes in NANMN was close to, but not significant (P = 0.13). The average flow of NANMN at pH <5.8 (n = 14) was 325 g N/d compared with 241g N/d at pH >5.8 (n = 162), with residuals showing some degree of lack of fit at low pH. However, the small number of data at pH <5.8 limits the power of the test. The EMPS (P < 0.0001; pR² = 0.37), OMTDR (P < 0.0001; pR² = 0.31), and DMI (P < 0.0001; pR² = 0.19) explained 87% of the variation in MN. The RUPI (P < 0.0001; pR² = 0.27), EMPS (P < 0.0001; pR² = 0.21), OMI (P < 0.0001; pR² = 0.09) and NI (P = 0.01; pR² = 0.01) explained 58% of the variation in NAN flow. The limited in vivo data does not allow to confirm the hypothesis that pH affects the flow of NANMN, but suggests that its flow is increased at low pH.