  Using spectroscopic data from the Deep Extragalactic Evolutionary Probe
(DEEP) Groth Strip survey (DGSS), we analyze the gas-phase oxygen abundances
for 56 emission-line field galaxies in the redshift range 0.26<z<0.82. Oxygen
abundances relative to hydrogen range between 8.4<12+log(O/H)<9.0 with typical
uncertainties of 0.17 dex. The 56 DGSS galaxies collectively exhibit a
correlation between B-band luminosity and metallicity, i.e., an L-Z relation.
Subsets of DGSS galaxies binned by redshift also exhibit L-Z correlations but
with different zero points. Galaxies in the highest redshift bin (z=0.6-0.82)
are brighter by ~1 mag compared to the lowest redshift bin (z=0.26-0.40) and
brighter by ~1-2 mag compared to local (z<0.1) field galaxies. This offset from
the local L-Z relation is greatest for objects at the low-luminosity (M_B ~
-19) end of the sample, and vanishingly small for objects at the
high-luminosity end of the sample (M_B ~ -22). Thus, both the slope and zero
point of the L-Z relation appear to evolve. Either the least-luminous DGSS
field galaxies have faded by 1--2 mag due to decreasing levels of star
formation, or they have experienced an increase in the mean metallicity of the
interstellar medium by factors of 1.3--2 (0.1-0.3 dex). The relatively greater
degree of luminosity and metallicity evolution seen among the lower luminosity
(sub L*) galaxies in the last 8 Gyr implies either a more protracted assembly
process, or a more recent formation epoch compared to more luminous L*
galaxies. (abridged)
