Activities of rat intestinal fucosyl-transferase (GDP-fucose: glycoprotein fucosyl-transferase; EC 2.4.1.68) and sialyl-transferase (N-acetylneuraminyl-transferase; EC 2.4.99.1) respectively exhibited a significant increase following weaning and a steady decrease between birth and weaning. The variations of the two glycosyl-transferase activities with age could not be explained by the presence of inhibitory factors in microsomes or cytosol, nor were they due to a natural modification of the milk composition at the end of lactation. The increase in fucosyl-transferase activity that followed weaning was prematurely induced by early weaning. Prolonged nursing prevented the normal increase in fucosyl-transferase activity, and late weaning delayed the increase. The sensitivity to the modification of the time of weaning indicated a major effect of dietary changes, related to the introduction of a solid carbohydrate-rich diet, on the developmental pattern of this enzyme. However, a stress response could not be excluded after early weaning since the corticosterone level of the early weaned rats was enhanced as compared to that of suckling rats. For sialyl-transferase, early weaning caused a slightly greater than normal decrease in the activity, whereas prolonged nursing only weakly diminished the normal decrease in the activity. Moreover, late weaning had no effect on the sialyl-transferase activity. The age-related variations of this enzyme likely are due to factors independent of diet. Thus, the developmental variations of the fucosyl- and sialyl-transferases appear to be differently regulated.