Content of middle mass molecules (MMM) was studied in blood plasma of rats treated with pyrazidol, the known inhibitor of monoamine oxidase of the A type. A distinct increase in the MMM nonpeptide component was detected both in intact animals and in the rats kept under hypoxic conditions (9,000 m, 3 hrs) after three administrations of pyrazidol. Under conditions of intermittent hypoxia higher increase of the nonpeptide component as well as an increase of the MMM peptide component were detected. The MMM blood fraction was also elevated in the animals adapted to hypoxia. A possible role of the MMM fraction in the pyrazidol-produced adaptation effect during hypoxia is discussed.