The activity of lactase enzyme declines after weaning. This study was to investigate changes in the lactase expression in the whole gastrointestinal tract during the development and the possibility that this and activity can be induced by lactose. Expression of lactase protein in the gut of 1-12-weeks old rats was studied by immunocytochemistry. Possible induction was evaluated by immunohistochemical and biochemical techniques in 8-week-old rats after lactose challenge for seven days. Lactase immunoreactivity was detected only in the small intestine and it decreased 20% during the week after weaning. A steady level of 40% lower than in the sucklings was found in the adult rats. In the lactose-challenged rats the optical density of immunoreactivity increased by about 30% in those that consumed the highest concentration of lactose. In the proximal jejunum, elevation of the enzymatic activity was three-fold. In the rat lactase protein expression decreased rapidly after weaning and expression and activity were induced by lactose-rich diet, most notably in the proximal jejunum.