The influence of growth rate of lucerne was investigated on the content of crude protein, NDF, ADF and the in situ degradability of crude protein and organic matter in the rumen of bulls. The content of crude protein decreased significantly (P < 0.05) with the increasing growth rate of lucerne, simultaneously the part of soluble N as per cent of total N decreased also. The effective degradability of crude protein decreased about 10% and the same value of organic matter about 5% in relation to the increasing content of NDF and ADF (P < 0.01) in dependence of the increasing growth rate of lucerne. The digestibility of non degraded crude protein in the rumen was investigated in vitro with pepsin, trypsin and chymotrypsin. The highest value resulted with 75.0% digestibility for this crude protein fraction in the stage of the start of flowering of lucerne.