We investigate the effects of temperature-dependent permeability in a hydrothermal upflow zone on the evolution of a seafloor hydrothermal system. Our mathematical modeling of a seafloor hydrothermal system with temperature-dependent permeability suggests that the system can be in one of two stable regimes of heat transfer. In one regime, heat conduction and thermoelastic effects do not play a significant role because the rock thermal expansion coefficient is too low or the rock porosity is too high. In the other regime, thermoelastic stresses reduce the permeability by orders of magnitude, and some fraction of the heat is transferred by conduction. In both regimes, essentially the same amount of heat is transported by the upflow but with different flow parameters. When thermoelastic stresses reduce the permeability, discharge occurs at relatively high temperature and low flow velocity, whereas when thermoelastic stresses have little effect on the permeability, discharge occurs at much lower temperature and higher flow velocity. The existence of two stable states of the hydrothermal system results from the nonlinearity of the dependency of permeability upon temperature and from specifying the heat flux entering the system. Consequently, a hydrothermal system can be on one or another solution branch, depending on small variations in system parameters, its history, and boundary conditions. At the top of the hydrothermal system the temperature difference between the branches can reach several hundred degrees. Relatively small changes in basal heat flux or rock permeability in the upflow zone can rather quickly switch the system from one stable branch to another. Such permeability changes may result from magmatic events, earthquakes, or chemical dissolution or precipitation. The calculations show that it is easier to drive the system from the high-temperature branch to the low-temperature one than vice versa. ¿ 2001 American Geophysical Union