Selenoprotein W (SelW) and selenium (Se) plays important roles in gastrointestinal function and that SelW expression in the gastrointestinal system of mammals is sensitive to Se levels. However, little is known about the pattern of SelW expression in the bird gastrointestinal tract. To investigate the distribution of SelW and effects of dietary Se levels on the SelW mRNA expression in the gastrointestinal tract tissues of birds, 1-day-old male chickens were fed either a commercial diet or a Se-supplemented diet containing 1.0, 2.0, 3.0 or 5.0 mg/kg sodium selenite for 90 days. The gastrointestinal tract tissues (tongue, esophagus, crop, proventriculus, gizzard, duodenum, small intestine, cecum and rectum) were collected and examined for Se content and mRNA levels of SelW. The mRNA expression of SelW was detected in all tissues. The greatest increase in SelW mRNA levels was observed in the gizzard, whereas Se content was highest in the duodenum and small intestine. A significant increase in SelW mRNA levels was observed in the gastrointestinal tract tissues of chickens fed the diets containing 1-3 mg/kg sodium selenite while decreased SelW mRNA levels were observed in the esophagus, crop, proventriculus, gizzard, duodenum and cecum in chickens fed the diet containing 5 mg/kg sodium selenite. These data indicate that SelW is widely expressed in the gastrointestinal tract tissues of birds and the transcription of the SelW gene is very sensitive to dietary Se.