  We calculate the creation of electron--positron pairs in Gamma-Ray Bursts
(GRBs) resulting from the collision between scattered and outward moving
gamma-ray photons. The number of pairs exceeds the number of ambient medium
electrons encountered by the GRB ejecta up to ~ 10^{16} cm from the center of
explosion. The shock resulting from the interaction of the ejecta with the
pair-wind may brighten the afterglow synchrotron emission during the first few
minutes. Even without this effect, the peak intensity of the optical afterglow
increases with the density of the surrounding medium. Therefore, observations
of the optical flux at early times constrain the density of the circumburst
medium. If the electron and magnetic field energies behind the forward shock
sweeping-up the pair-wind and the circumburst medium are as inferred from fits
to the broadband afterglow emission at 0.5-100 days, then the current upper
limits on the optical counterpart emission, set by the ROTSE and LOTIS
experiments, indicate that the circumburst medium within 0.01 pc is less dense
than 100 cm^{-3} or, if a wind, corresponds to a progenitor mass-loss to wind
speed ratio below 10^{-6} M_sun/yr/(1000 km/s).
