  We derive in this paper Gaussian estimates for a general parabolic equation u
t -- a(x)u x x = r(x)u over R. Here a and r are only assumed to be bounded,
measurable and essinf R a > 0. We first consider a canonical equation
$\nu$(x)$\partial$ t p -- $\partial$ x $\nu$(x)a(x)$\partial$ x p + W
$\partial$ x p = 0, with W $\in$ R, $\nu$ bounded and essinf R $\nu$ > 0, for
which we derive Gaussian estimates for the fundamental solution: $\forall$t >
0, x, y $\in$ R, 1 / Ct^{1/2} e^{--C|T (x)--T (y)--W t| 2 /t } $\le$ P (t, x,
y) $\le$ C /t^{1/2} e^{--|T (x)--T (y)--W t| 2 /Ct }where T is a corrector
satisfying appropriate properties. We then show that any solution u of the
original equation could be divided by some generalized principal eigenfunction
$\Phi$ $\gamma$ so that p := u/$\Phi$ $\gamma$ satisfies a canonical equation.
As a byproduct of our proof, we derive Nash type estimates, that is, Holder
continuity in x, for the solutions of the canonical equation.
