Let d be a nilpotent derivation of a semiprime ring R. It is proved th
at the subring of constants R(d) = {r is-an-element-of R \ d(r) = 0} i
s non-nilpotent. The result is applied to algebraic derivations of sem
iprime algebras and to the actions of certain finite dimensional Lie a
lgebras on semiprime associative algebras.