The aim of this experiment was to investigate total tract apparent digestibility of energy and nutrients in pigs previously fed a diet supplemented with 20% grass meal (high-fibre diet). Thirty-six pigs of 25 kg BW were randomly allotted to 4 treatment groups. The pigs were fed ad libitum a basal diet (B) or a high-fibre diet (HF) during the entire experiment (control group C and group HF105, respectively), or a high-fibre diet up to 50 or 80 kg BW (groups HF50 and HF80, respectively) followed by diet B up to 105 kg BW. Diet B contained (in g·DM^-1): crude protein, 212; crude fibre, 43; total lysine, 11.1 and gross energy, 18.0 MJ. Diet HF was formulated by mixing diet B with 20% of grass meal and contained (in g·DM^-1): crude protein, 191; crude fibre, 85; total lysine, 9.1 and gross energy, 18.3 MJ. The apparent digestibility of energy and nutrients was measured within a short time after exchanging diet HF for diet B (at approximately 55 and 85 kg BW) using an indirect method (with chromic oxide as an indicator).The apparent digestibility of energy and nutrients of diet HF was on average approximately 10% lower (P<0.01) than in diet B. Consequently, the energy value of diet HF was 1.67 MJ DE lower than in diet B. The digestibilities of energy and nutrients of diet B fed after the diet with a high fibre content (HF50 and HF80 groups) were not affected by the preceding diet, with the exception of the digestibility of crude fibre, NDF and ADF fractions, which were digested approximately 2.0% better, and ether extract, which was digested 3.2% worse (non significant difference). When the digestibilities of energy and nutrients in pigs weighing 55 and 85 kg were compared, however, it was found that heavier pigs digested (P<0.05) energy and protein (average by 1.0 and 1.9%) and crude fibre, NDF and ADF (by 8.2, 2.7 and 4.5%, respectively) more efficiently than pigs at 55 kg BW. Consequently, the energy value of diets fed to pigs at 85 kg BW was greater by 145 kJ·DE^-1 (P>0.05) than at 55 kg BW.