Fish were maintained in sea water equilibrated with a gas mixture containing a non-equilibrium mixture of the three molecular species of oxygen, O18-O18 (mass 36), O18-O16 (mass 34), and O16-O16 (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 secretory process and that therefore no exchange of atoms between oxygen molecules had occurred. Scission of the oxygen-oxygen bond probably does not occur during the transport process.

It is concluded that the active transport of oxygen into the swim-bladder by the gas gland is a transport of molecular oxygen.

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