1. 1. Volume regulation following shrinkage due to application of hyperosmotic conditions cannot be demonstrated in isolated axons of C. maenas during the time course of the experiments performed. 2. 2. The volume response observed in hypo-osmotic conditions can be dissociated into two distinct phases. Control of the intracellular amount of K + and, to a smaller extend, of NPS and “peptides” is implicated in a first, rapid L.B. phase of volume swelling limitation. The subsequent, slow volume readjustment phase can be essentially related to a regulation of the level of NPS and “peptides”.