  We have studied the evolution of primordial black holes (PBHs) in a universe
with a variable gravitational constant and bulk viscosity. We have found that
the strength of gravity has changed appreciably in the early universe. The
gravitational constant attained its greatest value at t=10^{-23} sec after the
Big Bang. PBHs formed at the GUT and electroweak epochs would have masses about
$1.85\times 10^9 g$ and $1.8\times 10^{-7} g$ respectively. Their temperatures
when they explode are $6\times 10^8K$ and 6 K respectively. PBHs formed during
nuclear epoch are hard to detect at the present time. The gravitational
constant ($G$) is found to increase as $G\propto t^2$ in the radiation epoch.
The gamma rays bursts (GRBs) may have their origin in the evaporation of the
PBHs formed during GUT time.
