  Nuclear reactions in stars occur between nuclei in the high-energy tail of
the energy distribution and are sensitive to possible deviations from the
standard equilibrium thermal-energy distribution. We are able to derive strong
constraints on such deviations by using the detailed helioseismic information
of the solar structure. If a small deviation is parameterized with a factor
exp{-delta*(E/kT)^2}, we find that delta should lie between -0.005 and +0.002.
However, even values of delta as small as 0.003 would still give important
effects on the neutrino fluxes.
