  Variability on time scales \delta t < t is observed in many gamma-ray burst
afterglows. It is well known that there should be no such variability if the
afterglow is emitted by external shock, which is produced by the interaction of
ultrarelativistic ejecta with the ambient interstellar medium, within the
framework of simple models. The corresponding constraints were established by
Ioka et al. (2005) and in some cases are inconsistent with observations. On the
other hand, if the motion is not relativistic, then the fast variability of the
afterglow can be explained much more easily.
  In this connection we discuss various estimates of the time of the transition
to subrelativistic motion in GRB source. We point out, that this transition
should occur on an observed time scale of ~10 days. In the case of a higher
density of the ambient interstellar medium ~10^2-10^4 cm^{-3} or dense stellar
wind with \dot M ~ 10^{-5} - 10^{-4} M_\odot/year the transition to a
subrelativistic motion can occur on a time scale of ~1 day. These densities may
well be expected in star-forming regions and around massive Wolf-Rayet stars.
