The blood and tissues of third instar Calliphora larvae contain a glucose dehydrogenase system which shows maximum activity at the crop-full stage and declines as pupation is approached. This decline is prevented by destruction of Weismann's ring at 3 days. It is suggested that dehydrogenase activity is the cause of the unstable reducing power of the blood and consequent inactivation of tyrosinase. The liberation of the 'pupation hormone' coincides with the complete and abrupt termination of dehydrogenase activity which leads to tyrosinase activity and hardening of the larval cuticle to form the puparium. Weismann's ring is therefore not merely active immediately before puparium formation, but exercises a regulatory influence on developmental processes during the life of the larva. Although no direct evidence is available of the existence in blowfly larvae of a juvenile hormone such as that found in Rhodnius by Wigglesworth, the results obtained seem consistent with the presence and later elimination of such a hormone.