The steady-state operating conditions and stability against temperature-density excursions are evaluated for tokamaks operating in the reactor regime on a D-T cycle. Global ion and alpha-particle balances and global ion and electron energy balances, together with MHD equilibrium and stability constraints, constitute the basic computational model. Constant, neoclassical, neoclassical-diffusion and trapped-ion instability scaling for particle and energy confinement times are considered.Steady-state operating conditions tend to be unstable below about 30 keV for constant confinement time scaling, assuming equal particle and energy confinement times. These low-temperature conditions are somewhat more unstable with neoclassical scaling and somewhat less unstable with trapped-ion instability scaling. The unstable modes, at low temperatures, are identified as pure temperature fluctuations for constant and neoclassical diffusion scaling, but as mixed temperature density fluctuations for trapped ion instability scaling. At higher temperatures, the least stable mode is a pure density fluctuation, for all confinement time scaling laws considered.Increasing the particle confinement time relative to the energy confinement time causes a build-up of alpha particles, with attendant bremsstrahlung enhancement, and a reduction in stability against temperature-density excursions.