1. 1. Changes in the hydrolyzing activities on acetylcholine and a series of triglycerides were investigated in larvae, pharate adults and emerged adults of Ceratitis capitata, Dacus oleae and Musca domestica. 2. 2. Triacetin, tributyrin and tricaproin were the only triglycerides split by the insect homogenates, the order of activity being adults > larvae > pharate adults. 3. 3. These three developmental stages of M. domestica exhibited higher activities on triglycerides and acetylcholine than did other Diptera. 4. 4. The order of activity on acetylcholine was adults > pharate adults > larvae . 5. 5. The inhibition of the hydrolyzing activities of C. capitata on triglycerides by eserine permits the suggestion that the existence of a change in the enzyme activity during development shows a tendency for it to be considered as a true esterase.