Ruminal degradation characteristics of rape seed and of untreated and formaldehyde-treated rapeseed meals containing less than 8 µmol of total glucosinolates per gram of dry matter were investigated. Feeds were incubated in polyester bags in the rumen of three fistulated steers for 0, 2, 4, 8, 16, 24 and 48 h. The rate and extent of degradation of dry matter and crude protein and disappearance of amino acids from polyester bags were determined. The crude protein was more degradable than the dry matter of rape seed and untreated rapeseed meal, whereas the opposite was observed for the formaldehyde-treated rapeseed meal. The treatment of the rapeseed meal with formaldehyde greatly reduced the rate and extent of dry matter and crude protein degradation compared with the untreated meal. Effective degradabilities of dry matter and crude protein declined in the following order: rape seed > untreated rapeseed meal > formaldehyde-treated rapeseed meal. The effective degradabilities of crude protein at assumed ruminal passage rates of 2, 5 and 8%/h ranged from 77 to 62% for the rape seed, 76 to 51% for the untreated meal and 39 to 16% for the formaldehyde-treated meal. With increasing exposure time to the rumen, the amino acid profile of undegraded residues of rape seed and of untreated rapeseed meal changed. As a result, large differences among amino acids were observed for percentages of disappearance from the bags. Treatment of rapeseed meal with formaldehyde was effective in preserving the amino acid composition of the undegraded feed protein.