In this paper we prove that any correspondence from a smooth algebraic
variety to itself which is algebraically equivalent to zero is a
nilpotent in the ring of correspondences modulo rational equivalence
(with rational coefficients). In fact we prove a stronger result which
shows that any cycle algebraically equivalent to zero is
``smash-nilpotent'' modulo rational equivalence.
The last section of the paper is purely speculative and contains a
discusion of a very strong Nilpotence Conjecture for algebraic cycles.