  We study a parity-time (PT) symmetric ring lattice, with one pair of balanced
gain and loss located at opposite positions. The system remains PT-symmetric
when threaded by a magnetic flux; however, the PT symmetry is sensitive to the
magnetic flux, in the presence of a large balanced gain and loss, or in a large
system. We find a threshold gain/loss, above which, any nontrivial magnetic
flux breaks the PT symmetry. We obtain the maximally tolerable magnetic flux
for the exact PT-symmetric phase, which is approximately linearly dependent on
a weak gain/loss.
