In a previous paper some effects of anisosmotic and unbalanced salt solutions ontheluminescent responses of Chaetopterus variopedatus have been described (Nicol, 1952). Similar experiments have been carried out on polynoid elytra, andthe results are described elsewhere in this Journal, together with a brief review ofthe relevant scientific literature (Nicol, 1954b). The present experimentswere undertaken to investigate the effects of single salts in physiological concentrations, balanced by the addition of some inert substance, and to explain the possible mechanism of stimulation by hypo-osmotic solutions.