The present study aimed to establish the effects of grass meal and duration of feeding it on the carcass chemical composition, and on the intramuscular fat (IMF) concentration and fatty acid composition of the musculus longissimus dorsi (MLD) in 48 pigs growing from 25 to 105 kg body weight (BW). The pigs were fed a commercial diet (C) or a diet containing 20% grass meal (GM). Changes of the chemical components in the carcass and intramuscular fat of the MLD were estimated using the comparative slaughter method. The animals were slaughtered at 50, 80 and 105 kg BW. Pigs consuming the grass meal diet grew more slowly (by 9.2%; P = 0.007), but they had similar amounts of protein, ash and water (average 9.9, 1.8 and 35.9 kg, respectively) and less (by 18.4%; P = 0.001) fat in the carcass compared with those fed the commercial diet. Pigs fed the GM diet had approximately 20% less IMF (P = 0.003), lower (P = 0.024) concentration of saturated fatty acids (SFA) and higher (P = 0.047) concentration of polyunsaturated fatty acids (PUFA) in the MLD than pigs fed the C diet. The GM pigs also had a higher concentration of C18:3 n-3 (linolenic acid; P = 0.047) and tended to have a lower (P = 0.091) C18:2 n-6/C18:3 n-3 ratio in the MLD than C pigs. The PUFA/ SFA ratio remained low, but was more beneficial in the pigs fed the GM diet compared with those fed the C diet (0.40 vs 0.35, respectively; P = 0.102).