  We show that if the sources of ultra-high energy cosmic rays (UHECRs) with
energies E>10^{19}eV are associated with galaxies, then the production of
UHECRs must occur in transient events. Our galaxy is currently at a dim state
in between transients. The time interval between transients is >3x10^4 years if
the minimum Lorentz factor to which protons are accelerated in the transients
is between 10^3 and 10^7. This constraint is satisfied by gamma-ray bursts.
