The synchrotron radiation spectrum in the extraordinary mode, emitted by highly energetic electrons in the midplane of an axisymmetric mirror end cell of the TARA tandem mirror machine, has been measured up to about the 40th harmonic of the electron cyclotron frequency using a fast scanning Michelson interferometer. The temporal evolution of the perpendicular (to the axial magnetic field) temperature and number density of hot electrons has been determined. Hot electrons, with a peak temperature of ∼ 330 keV and a peak density of ∼2% of the background plasma density, contribute significantly to the plasma pressure in the axisymmetric end cell.