  Fundamental Plane studies provide an excellent means of understanding the
evolutionary history of early-type galaxies. Using the Low Resolution Imaging
Spectrograph on the Keck telescope, we obtained internal stellar kinematic
information for 36 field galaxies in the Groth Strip--21 early-type and 15 disk
galaxies. Their redshifts range from 0.3--1.0, with a median redshift 0.8. The
slope of the relation shows no difference compared with the local slope.
However, there is significant evolution in the zero-point offset; an offset due
to evolution in magnitude requires a 2.4 magnitude luminosity brightening at
z=1. We see little differences of the offset with bulge fraction, which is a
good surrogate for galaxy type. Correcting for the luminosity evolution reduces
the orthogonal scatter in the Fundamental Plane to 8%, consistent with the
local scatter. This scatter is measured for our sample, and does not include
results from other studies which may have different selection effects. The
difference in the degree of evolution between our field sample and published
cluster galaxies suggests a more recent formation epoch--around z=1.5 for field
galaxies compared to z>2.0 for cluster galaxies. The magnitude difference
implies that the field early-type galaxies are about 2 Gyr younger than the
cluster ellipticals using standard single-burst models. However, the same
models imply a significant change in the rest-frame U-B color from then to
present, which is not seen in our sample. Continuous low-level star formation,
however, would serve to explain the constant colors over this large magnitude
change. A consistent model has 7% of the stellar mass created after the initial
burst, using an exponentially decaying star formation rate with an e-folding
time of 5 Gyr.
