Mechanisms of Hormone Action: A NATO Advanced Study Institute focuses on the action mechanisms of hormones, including regulation of proteins, hormone actions, and biosynthesis. The selection first offers information on hormone action at the cell membrane and a new approach to the structure of polypeptides and proteins in biological systems, such as the membranes of cells. Discussions focus on the cell membrane as a possible locus for the hormone receptor; gaps in understanding of the molecular organization of the cell membrane; and a possible model of hormone action at the membrane level. The text also ponders on insulin and regulation of protein biosynthesis, including insulin and protein biosynthesis, insulin and nucleic acid metabolism, and proposal as to the mode of action of insulin in stimulating protein synthesis. The publication elaborates on the action of a neurohypophysial hormone in an elasmobranch fish; the effect of ecdysone on gene activity patterns in giant chromosomes; and action of ecdysone on RNA and protein metabolism in the blowfly, Calliphora erythrocephala. Topics include nature of the enzyme induction, ecdysone and RNA metabolism, and nature of the epidermis nuclear RNA fractions isolated by the Georgiev method. The selection is a valuable reference for readers interested in the mechanisms of hormone action.