THE object of this communication is to present the hypothesis that hormones modify the ‘cytoskeleton’ of the cell. Though I have held this view for some time, and have discussed it with others, I have not advanced it formally, because until now it did not appear to be useful. There are, it is true, subcellular actions which have been described for hormones ; notably one may mention the interesting effects of thyroxine upon the oxidative phosphorylation of mitochondria which formed the basis of a recent lecture to the Third International Congress of Biochemistry by C. Martius. There are also conclusive observations upon the action of insulin in altering permeability to sugars1,2; and also, on the contrary, the widespread idea that insulin controls the formation of glucose 6-phosphate. Yet in spite of interest in these facts, there is a general feeling that hormones act upon the cell as a whole, and that their action is somewhat diverse rather than being upon one enzymic or other step (cf. A. Beloff-Chain et al. 3, and see particularly the discussion of this by O. Hechter4). It is from this aspect that I think the ‘cytoskeletal’ view may prove to be a useful working hypothesis.