In Bombyx 5th instar larvae, haemolymph trehalose concentrations were maintained in a range of 8-14 mM during the feeding period and then decreased to approximately half the level concomitantly with the occurrence of a gut purge. The decrease occurred as well in larvae that were starved for 29 hr before the gut purge and those that were ligated around the neck. In contrast, ligation between the thorax and abdomen suppressed such a decrease in trehalose concentrations. These results indicated that thoracic factor(s) was involved in the decrease in the haemolymph trehalose concentration. Injections of ecdysteroids into the isolated abdomens of day 5 larvae resulted in a decrease in haemolymph trehalose concentrations. These findings suggest that an increase in the haemolymph ecdysteroid titer at the wondering stage brings about a decrease in the haemolymph trehalose concentrations. Ecdysteroids thus appear to play an important role in the changes in the carbohydrate metabolism at the larval-pupal transformation.