  The recent detection of an unusually hard spectral component in GRB941017
extending to $\ge200$ MeV is hard to explain as a synchrotron emission from
shock-accelerated electrons.
  It was argued to imply acceleration of protons to ultra-high energy. We show
here that the "high energy tail" can be explained as emission from
shock-accelerated electrons in the early afterglow epoch, taking into account
the effect of synchrotron self-absorption. High energy observations set in this
case stringent constraints on model parameters: A lower limit to the total
explosion energy $E\gsim5 \times 10^{53}$ erg (assuming spherical symmetry);
  An upper limit to the density of gas surrounding the explosion,
  $n\lsim10^{-2}(E/10^{54}{\rm erg}){\rm cm}^{-3}$; A lower limit to the
expansion Lorentz factor $\Gamma_i\gsim 200$; and An upper limit to the
fraction of thermal energy carried by the magnetic field behind the shock
driven into the surrounding medium,
  $\epsilon_{B,f} <= 10^{-4}$. Such constraints can not be inferred from
keV--MeV data alone. The unusually low value of $\epsilon_{B,f}$ and the
unusually high ratio $E/n$ may account for the rareness of GRB941017-type high
energy tails. Tighter constraints on model parameters may be obtained in the
future from optical and sub-TeV observations.
