The distribution of ecdysone before and during puparium formation has been determined in various tissues of Calliphora. The hormone level in the blood remains relatively constant from 20 hours before pupation to 5 hours after pupation. Ecdysone first appears in the integument 8 — 5 hours before pupation reaching a maximum of 27.5% of the total hormone content in the body 5 hours after pupation. The titer in the fat body is low until 5 hours after pupation when it rises sharply to 48% of the total body content. Exogenous ecdysone injected into isolated abdomens is rapidly inactivated. The activity declines exponentially with a half-life of about 3 hours.