We compare the electronic temperatures and the population inversion both below and above the lasing threshold in three quantum-cascade lasers (QCLs) operating at 2.8 THz, 3.2 THz, and 3.8 THz using microprobe band-to-band photoluminescence. In the lasing range, while the ground-state temperature remains close to the lattice one (90 K–100 K), the upper radiative state heats up to ∼ 200 K. From the measured thermal resistance and the power dependence of the ground-state electronic temperature, we get a value of the electron-lattice energy relaxation rate comparable with that typical of midinfrared QCLs.