Summary: Gases which are customarily viewed as biologically inert may nevertheless be secreted into the swimbladder at high pressure just like their biologically active counterparts, O2 and CO2. The late Werner Kuhn proposed that the mechanism for all gases was countercurrent multiplication of small changes in partial pressure initiated by secretions of the gas gland situated near the bend of the multiplier loop. Hitherto this hypothesis has been confirmed only for O2 and CO2: an analysis of swimbladder data on argon/nitrogen ratios presented here gives the required confirmation in the inert gas case. A second part of this paper deals with the nature of the secretory agent relevant to inert gas. Kuhn postulated the secretion of an electrolyte to salt out dissolved gas in addition to lactic acid secretion known to be operative in the release of O2 and CO2. This postulate may be redundant: lactic acid is shown to release N2 from fish red cells perhaps by a pH‐related change in haemoglobin polar groups. Countercurrent multiplication of all gases would thus be under the control of a single agent. Interspecies variations in swimbladder gas content would not be inconsistent with this unitary theory since these may merely reflect interspecies variations in the development of the multiplier apparatus and probable variations in the pH‐effect also.