This study deals with the role of insulin in the regulation of the intestinal glycoprotein fucosylation process during postnatal development in the rat. Circulating insulin level was found to increase at weaning time in parallel with -1,2-fucosyltransferase activity and with the appearance of -1,2-fucoproteins in brush-border membranes. Insulin treatment of young suckling rats induced a precocious increase in fucosyltransferase activity and in the biosynthesis of its substrate (GDP-fucose), but the sensitivity to insulin disappeared after weaning. The insulin level was lower in 22-day-old rats that received prolonged nursing (on a high-fat diet) compared with age-matched normally weaned rats (on a high-carbohydrate diet), whereas the appearance of -1,2-fucoproteins and the increase in activity of -1,2-fucosyltransferase were delayed, as was the decrease in the degradation of GDP-fucose. In 22-day-old animals that received prolonged nursing and insulin treatment, the -1,2-fucosyltransferase activity reached a level close to that observed in age-matched weaned rats, and several -1,2-fucoproteins appeared in brush-border membranes with a molecular mass similar to that found in weaned rats. These results suggest that changes in insulin levels at weaning time (as caused, in the present case, by dietary modifications) may be responsible for the regulation of the glycoprotein fucosylation process, essentially by increasing fucosyltransferase activity.