Fish were maintained in sea water equilibrated with a gas mixture containing a non-equilibrium mixture of the three molecular species of oxygen, O18-O 18 (mass 36), O18-O t6 (mass 34), and Ole-O 16 (mass 32). Analyses in the mass spectrometer, of the gases secreted into the swim-bladder showed that no change in the relative abundance of these three molecular species had occurred during the secr...