  Given a finite dimensional Lie algebra $L$ let $I$ be the augmentation ideal
in the universal enveloping algebra $U(L)$. We study the conditions on $L$
under which the $Ext$-groups $Ext (k,k)$ for the trivial $L$-module $k$ are the
same when computed in the category of all $U(L)$-modules or in the category of
$I$-torsion $U(L)$-modules. We also prove that the Rees algebra $\oplus _{n\geq
0}I^n$ is Noetherian if and only if $L$ is nilpotent. An application to
cohomology of equivariant sheaves is given.
