  We calculate the decay rate for a state prepared in a thermal density matrix
centered on a metastable ground state. We find a rate that is intrinsically
time {\it dependent}, as opposed to the {\it constant} rates of previous works.
The rate vanishes at early times, rises to a maximum and eventually falls-off
to zero as a consequence of unitary time evolution. Finally, we discuss
extensions of this calculation to field theories and possible implications for
both sphaleron mediated transitions and first order inflationary theories.
