Sixty Holstein heifers (30 days in milk) were used to examine the effects of protected methionine on milk production, milk composition and reproduction performance up to 100 days post-partum. Heifers were randomly divided into two dietary treatment groups (30 each) receiving either 14 or 16 % crude protein. Heifers of each treatment group were assigned into three subgroups with a daily supplementation of 0, 15 or 25 g/hd/d of rumen protected methionine. Diet fed was a total mixed ration consisting of fresh alfalfa, corn silage and a concentrate mixture. In addition, alfalfa hay was offered ad libitum during the whole experimental period.