  We demonstrate a hybrid method based on field ionization and state-selective
de-excitation capable of measuring the lifetimes of high-lying Rydberg states.
For nS Rydberg states of Rb atoms with principal quantum number $60\leq
n\leq88$, we measure both the individual target state lifetimes and those of
the ensemble of Rydberg states populated via black-body radiation-induced
transitions. We find good overall agreement with numerical calculations of the
expected lifetimes in both cases. However, for the target state lifetimes, we
find a local deviation towards shorter lifetimes for states around $n=72$,
which we interpret as a signature of a modified black-body spectrum in the
finite volume in which our experiments take place.
