We present Gran-Telescopio-Canarias/OSIRIS optical spectra of 4 of the most compact and massive early-type galaxies in the Groth Strip Survey at redshift z 1, with effective radii R_e=0.5-2.4 kpc and photometric stellar masses M_⋆=1.2-4×10^11 M_⊙. We find these galaxies have velocity dispersions σ=156-236 km s^-1. The spectra are well fitted by single stellar population models with approximately 1 Gyr of age and solar metallicity. We find that: (i) the dynamical masses of these galaxies are systematically smaller by a factor of 6 than the published stellar masses; (ii) when estimating stellar masses as 0.7×M_{dyn}, a combination of passive luminosity fading with mass/size growth due to minor mergers can plausibly evolve our objects to match the properties of the local population of early-type galaxies.