  We report on the discovery of PSR J1141-6545, a radio pulsar in an eccentric,
relativistic 5-hr binary orbit. The pulsar shows no evidence for being
recycled, having pulse period P = 394 ms, characteristic age tau_c = 1.4 x 10^6
yr, and inferred surface magnetic dipole field strength B = 1.3 x 10^12 G. From
the mass function and measured rate of periastron advance, we determine the
total mass in the system to be (2.300 +/- 0.012) solar masses, assuming that
the periastron advance is purely relativistic. Under the same assumption, we
constrain the pulsar's mass to be M_p < 1.348 solar masses and the companion's
mass to be M_c > 0.968 solar masses (both 99% confidence). Given the total
system mass and the distribution of measured neutron star masses, the companion
is probably a massive white dwarf which formed prior to the birth of the
pulsar. Optical observations can test this hypothesis.
