In two experiments, lactating cows (26 in exp. 1; 32 in exp. 2) were fed an 11.3% crude protein (CP) diet, a 14.7% CP diet containing untreated soybean meal (SBM) or a 14.6% CP diet containing formaldehyde-treated SBM (FSBM), all three fed as a complete blended diet with control corn silage (CCS) (exp. 1); a 12% CP diet, a 13.4% CP diet containing SBM or a 13.3% CP diet containing FSBM, all fed as a complete blended diet with urea-treated corn silage (UCS) (exp. 2). In both experiments concentrates and CCS or UCS were mixed (50:50 dry matter) daily and fed ad lib for a 16-wk period. The increase of CP over the low CP negative control by addition of untreated SBM resulted in a significant (P < 0.05) increase in milk production in both experiments. The addition of FSBM had no significant effect. When comparing solids-corrected milk (SCM), addition of SBM, whether treated or not, significantly increased SCM in both experiments. In neither experiment did FSBM improve milk production over untreated SBM. All animals fed CCS lost body weight but there was no difference between treatments. For animals fed UCS the low CP group lost significantly (P < 0.05) more body weight than did the group fed FSBM. Dry matter (DM) intake was significantly (P < 0.05) lower for the FSBM fed group than for the group fed untreated SBM in exp. 1 (CCS). DM intake was significantly (P < 0.05) increased by either FSBM or SBM over the low CP control fed UCS. Milk protein was the only component significantly (P < 0.05) decreased by FSBM in exp. 1. Isobutyric and isovaleric acids were significantly (P < 0.05) lower in rumen fluid of animals on FSBM plus CCS. Rumen microbial protein content seemed to be more adversely affected by FSBM with CCS than UCS. Amino acid content of FSBM and SBM demonstrated that lysine and tyrosine were lost from SBM after treatment with formaldehyde. The lack of a response in milk production to FSBM is discussed in terms of a lysine and tyrosine deficiency for milk protein synthesis and the adverse effect that formaldehyde protection of SBM may have on the rumen microbial population. Key words: Lactation, dairy cows, urea, corn silage, soybean meal, formaldehyde treatment