This study was conducted to determine the effects of soybean meal (SBM) treated with formaldehyde on rumen fermentation pattern, microbial protein synthesis, amount of by-pass protein, and amino acid absorption from small intestines. Four 1-year-old rams with ruminai, duodenal, and ileal cannulas were fed 800 g of dry clover plus a concentrate mixture containing 100 g of SBM treated with 0%, 0.3%, 0.6%, and 0.9% formaldehyde. The amount of crude protein passed through the duodenum was not affected by the formaldehyde treatment; however, the amount of microbial protein decreased in rams fed with SBM treated with both 0.3% and 0.6% formaldehyde (P < 0.05). Digestibility of acid detergent fibre (ADF) and neutral detergent fibre (NDF) in the duodenum and ileum did not differ by treatments. In the rams fed SBM treated with both 0.3% and 0.6% formaldehyde, absorbability of arginine, histidine, leucine, methionine, phenylalanine, and valine (essential amino acids), as well as alanine and aspartic acid (non-essential amino acids) significantly increased, whereas tyrosine absorbability notably decreased (P < 0.05). Formaldehyde treatment increased the absorbability of other essential amino acids (leucine and valine) and non-essential amino acids (serine and tyrosine) in the ileum (P < 0.05). Increasing the percentage of formaldehyde linearly increased total essential amino acid absorption in the duodenum (P < 0.05). In conclusion, 0.3% and 0.6% formaldehyde treatment enhanced the bioavailability of protein in SBM. © TÜBİTAK.