Cholinesterase activity is detectable in the Japanese quail embryo, in the yolk and subembryonic liquid, but not in the albumen. Obviously, this enzyme is deposited by the hen into the yolk and from there it is transferred to the subembryonic liquid. In contrast, in the embryo the enzyme is synthesized by itself and the amount increases with the age of the embryo. By using BW284c51 1,5-bis-(4-allyldimethylammoniumphenyl)pentan-3-one bromide and ISO-OMPA tetraisoprophylpyrophosphoramide as inhibitors, it was found that the enzyme in the embryo is predominantly acetylcholinesterase (EC 3.1.1.7), whereas that in the yolk and subembryonic liquid is butyrylcholinesterase (EC 3.1.1.8). Both types are inhibited by dichlorphos. However, the embryonic enzyme activity is restored within 8 hr, whereas that in the subembryonic liquid remained inactive at least for 72 hr after inhibition. Enzyme inhibition leads to retardation of the development, to reduced accumulation of glucose and amino acids in the subembryonic liquid and finally to death of the embryo, suggesting that the developmental retardation is due to the restricted supply of glucose and amino acids. Surprisingly, most of the embryos die when the embryonic enzyme activity has again been restored.