Numerical simulations have been performed to predict pressure transients in deep geothermal reservoirs at sub- and super-critical temperatures. First, pressure drawdown and buildup tests of reservoirs with different initial conditions were simulated. The calculated pressure responses are dominated by non-linear changes of fluid kinematic viscosity and compressibility. The pressure of a super-critical zone is shown to cause complex behavior. Short- and longterm production tests in both unbounded and bounded reservoirs were then simulated. Unbounded and bounded reservoirs exhibit very similar short-term production behavior near or above the critical temperature (375-400 degrees C). Unbounded reservoirs of low transmissivity (kh = 1 darcy-meter) exhibit long-term production behavior that depends on whether the reservoir is sub-critical (300-375 degrees C) or super-critical (400 degrees C); substantial increases in flowing enthalpy and declines in feedpoint pressure occur at early times in the super-critical reservoir.