The structural and chemical changes taking place in the cuticle of the mature larva of Sarcophaga as it is converted into the puparium are fully described. The formation of the hard and dark exocuticle of the puparium from the outer endocuticle of the larva is due, as Pryor (1940b) has shown, to the action of a phenol derived from the blood, a polyphenol oxidase located in the inner epicuticle and present considerably before pupation being responsible for the rapid oxidation of the phenol. The blood phenol, although spontaneously oxidizable, passes unchanged through the inner endocuticle which remains soft and white, and this may be correlated with the ability of the inner endocuticle to reduce methylene blue. The phenol hardening the puparium is produced in the blood by the enzymatic oxidation of tyrosine. Tyrosine in the blood increases steadily in amount before pupation, and during this period the enzyme tyrosinase also appears and increases. The source of tyrosinase appears to lie in the oenocytoids of the blood, which increase in number as the larva matures and die away shortly before pupation. But although tyrosine and tyrosinase are present together in the blood of the mature larva, inhibiting conditions prevent the activity of the enzyme until very shortly before pupation. Tyrosinase activity, however, may be experimentally stimulated before pupation by the action of methyl alcohol and more slowly by narcotics, and it is speculated that the inhibition of tyrosinase in the mature larva may perhaps be effected by the activity of a further enzyme, a dehydrogenase, which operates by increasing the reducing power of the blood. Colorimetric and potentiometric studies of the reducing power of the blood have been carried out, and it has been found that the oxidation-reduction potential of the blood decreases steadily as the larva matures, and then rises sharply just before pupation. The coincidence of this rise with the period of liberation of the pupation hormone suggests that one function of the hormone is to destroy the reducing power of the blood, so liberating tyrosinase activity and leading to the production of a polyphenol which passes into and hardens the cuticle. In conclusion, it is suggested that the homology between the insect and crustacean cuticles is closer than has been hitherto emphasized.