1. 1. Brain trehalase specific activity and trehalosemia were measured during the end of the developmental life cycle in non-diapausing and diapausing insects. 2. 2. During non-diapausing development, trehalosemia reached maximum values at the beginning of pupal life. Then a constant decrease was observed up to the end of adult life. 3. 3. The specific activity of brain trehalase was maximum when the insects were in active feeding periods, minimum activity appearing during moulting phases. 4. 4. During diapausing development, trehalosemia was very high at the beginning of pupal life, particularly when insects were exposed to wintering conditions. 5. 5. When diapause was broken, trehalosemia fell, announcing adult emergence. 6. 6. Brain trehalase activity showed the same qualitative variations as in non-diapausing larvae, but with rather lower values. 7. 7. During pupal life, brain trehalase activity decreased markedly during the long period necessary to obtain diapause breakdown. 8. 8. Wintering conditions allow a progressive increase of brain trehalase activity, which preceded the fall of trehalosemia.