The objective of this study was to evaluate lactating dairy cow ruminal fermentation and rumen content mass as affected by diets containing molasses (M) or finely ground dry corn grain at 3 levels of M (0, 5.25, 10.5% DM) and with differing levels of ruminally degradable protein (+RDP or –RDP). Twelve ruminally cannulated, multiparous Holstein cows (initial mean ± SD: parity 2.25 ± 0.62; 185 ± 56 DIM; 41.3 ± 6.3 kg milk) were allocated to treatment sequences in a split-plot, replicated 3x3 Latin square design with a 2x3 factorial arrangement of treatments. Experimental diets were individually fed to cows for ad libitum consumption and formulated to contain 35% corn silage and 20% alfalfa silage with +RDP formulated with a greater concentration of solvent soybean meal (SBM) and expellers SBM replacing a portion of solvent SBM in -RDP. Each period lasted 28 d; on d 21 and 28 of each period, rumen contents of all cows were evacuated, subsampled, and replaced beginning 6 h after feeding. On d 25–28 of each period, samples of rumen contents were taken at 4 or 6 h intervals to represent every 2 h. Data were analyzed in SAS with significance was declared at P < 0.05 and tendencies at 0.05 < P ≤ 0.10. Dry matter intake and production responses were not different among diets. Ruminal pH, organic acid concentration, and ammonia concentration were not affected by diet and followed similar temporal patterns. Ruminal acetate molar percent decreased linearly and butyrate molar percent increased linearly with increasing levels of M. Ruminal free AA concentration was greater for +RDP and branched chain VFA concentration declined linearly with increasing M. Rumen content mass was greater for –RDP, which was due to greater quantities of rumen liquid. Ruminal acetate pool size was greater for -RDP and butyrate pool size increased linearly with M. Lactate pool size tended to be higher with +RDP. Under the conditions of this experiment, ruminal fermentation or rumen content mass were affected by level of M or RDP individually, but these diet characteristics were not detected to interact with each other.