  We present the first results from a major Hubble Space Telescope program
designed to investigate the cosmological evolution of quasar host galaxies from
z~2 to the present day. Here we describe J and H-band NICMOS imaging of two
quasar samples at redshifts of 0.9 and 1.9 respectively. Each sample contains
equal numbers of radio-loud and radio-quiet quasars, selected to lie within the
same narrow range of optical absolute magnitude (-24 > M_V > -25). Filter and
target selection were designed to ensure that at each redshift the images
sample the same part of the object's rest-frame spectrum, avoiding potential
contamination by [OIII]lambda5007 and H-alpha emission lines. At z=1 the hosts
of both radio-loud and radio-quiet quasars lie on the same Kormendy relation
described by 3CR radio galaxies at comparable redshift. There is some evidence
for a gap of ~1 mag between the host luminosities of RLQs and RQQs, a
difference that cannot be due to emission-line contamination given the design
of our study. However, within current uncertainties, simple passive stellar
evolution is sufficient to link these galaxies with the elliptical hosts of
low-redshift quasars of comparable nuclear output, implying that the hosts are
virtually fully assembled by z=1. At z=2 the luminosity gap appears to have
widened further to ~1.5 mag. Thus while the hosts of radio-loud quasars remain
consistent with a formation epoch of z>3, allowing for passive evolution
implies that the hosts of radio-quiet quasars are ~2-4 times less massive at
z=2 than at low z.
