The transfer of Aeshna cyanea larvae from 5 mosmol sea salt solution, which is similar to freshwater and very hypo-osmotic to the haemolymph, to hyper-osmotic sea salt solution (400 mosmol) leads to a rapid increase in the osmolarity and the NaCI concentration of the haemolymph, which after 2 days exceed those of the external solution by about 60 mosmol and 20 mM, respectively. When the osmotic concentration of the external medium is raised to 400 mosmol by the addition of mannitol and the salinity left unchanged at 5 mosmol, the haemolymph concentration increases more slowly and becomes only iso-osmotic to the external medium after 5 days. In s o d i u m— or c h I o r i d e— substituted media the haemolymph concentration of the kind of ion, which is externally present, slowly increases, whereas the concentration of the externally substituted counterion slowly drops. The results suggest that sufficient concentrations of both sodium and chloride in the external medium are required for the proper osmoregulation.