  We derive the magnitude of fluctuations in total synchrotron intensity in the
Milky Way and M33, from both observations and theory under various assumption
about the relation between cosmic rays and interstellar magnetic fields. Given
the relative magnitude of the fluctuations in the Galactic magnetic field (the
ratio of the rms fluctuations to the mean magnetic field strength) suggested by
Faraday rotation and synchrotron polarization, the observations are
inconsistent with local energy equipartition between cosmic rays and magnetic
fields. Our analysis of relative synchrotron intensity fluctuations indicates
that the distribution of cosmic rays is nearly uniform at the scales of the
order of and exceeding $100\p$, in contrast to strong fluctuations in the
interstellar magnetic field at those scales. A conservative upper limit on the
ratio of the the fluctuation magnitude in the cosmic ray number density to its
mean value is 0.2--0.4 at scales of order 100\,pc. Our results are consistent
with a mild anticorrelation between cosmic-ray and magnetic energy densities at
these scales, in both the Milky Way and M33. Energy equipartition between
cosmic rays and magnetic fields may still hold, but at scales exceeding 1\,kpc.
Therefore, we suggest that equipartition estimates be applied to the observed
synchrotron intensity smoothed to a linear scale of kiloparsec order (in spiral
galaxies) to obtain the cosmic ray distribution and a large-scale magnetic
field. Then the resulting cosmic ray distribution can be used to derive the
fluctuating magnetic field strength from the data at the original resolution.
The resulting random magnetic field is likely to be significantly stronger than
existing estimates.
