The objective was to delineate the interactions between partial replacement of barley starch with lactose (as dried whey permeate, DWP) and dietary rumen-degradable protein (RDP) levels on milk production, ruminal pH, and nitrogen (N) utilization in cows. Eight Holstein cows (738 ± 98 kg BW; 93 ± 39 DIM) were used in a replicated 4 × 4 Latin square with a 2 × 2 factorial arrangement of dietary treatments. One square had 4 ruminally-cannulated cows for ruminal pH measurements. The treatment factors were levels of DWP (0 vs. 12.5% of DM) and RDP (9.5 vs. 11.5%). Diets contained ~17.7% CP, and the 0% and 12.5% DWP diets contained 23.7 and 17.8% starch, respectively. The dietary addition of DWP tended (P = 0.08) to increase DMI; however, actual and energy-corrected milk yields were not affected by the dietary addition of DWP or level of RDP. Milk fat content and yield were not affected by diet. The dietary addition of DWP increased milk protein content (P = 0.01) and yield (P = 0.01). Milk lactose content was similar in cows fed 9.5 and 11.5% RDP without added DWP, but milk lactose content was greater in cows fed 9.5% RDP compared with those fed 11.5% RDP with added DWP (interaction, P < 0.01). Milk urea-N content was similar in cows fed 9.5 and 11.5% RDP with added DWP, but milk urea-N content was greater in cows fed 11.5% RDP compared with those fed 9.5% RDP with added DWP (interaction, P < 0.01). Total-tract CP digestibility was greater (P = 0.02) in cows fed 11.5% RDP compared with those fed 9.5% RDP. Urinary N excretion was unaffected by diet; however, adding DWP increased (P = 0.03) fecal N excretion (g/d). Feeding DWP increased milk N secretion (P < 0.01). Total N excretion was unaffected by diet; however, apparent N balance tended to be greater (P = 0.10) and productive N (i.e., milk N + apparent N balance) was greater (P = 0.04) in cows fed 11.5% RDP compared with 9.5% RDP. Dietary treatments had no effect (P > 0.05) on mean ruminal pH, and area or duration when ruminal pH was less than 5.8 and 5.5. Our results indicate that partially replacing barley starch with DWP had no negative effects on ruminal acidosis, but it increased milk N secretion.