Evidence for previous periods of hydrothermal activity in Long Valley caldera exists in the form of extensive deposits of hydrothermal alteration products at several locations within the caldera and saline deposits in Searles Lake which contain mineral assemblages contributed by hot spring discharge from Long Valley. Hydrothermal activity was more intense in the past and probably involved fluid circulation to depths of several kilometers or more with heat supplied by the Long Valley magma chamber. During the past 40,000 years the heat source may have shifted to the Inyo-Mono magmatic system beneath the west moat, where deep fluid circulation supplied hot water to shallower zones of lateral flow within the Bishop Tuff beneath the resurgent dome. The present-day hydrothermal system in Long Valley appears to consist of two principal zones in which hot water flows laterally from west to east at depths of less than 1 km within and around the resurgent dome. Maximum measured temperatures within these zones are near 170°C, but estimates from chemical geothermometers and extrapolation of a high-temperature gradient measured in a recent drill hole indicate that a source reservoir at temperatures near 240° may exist at greater depths within the Bishop Tuff beneath the west moat. Regions possibly containing silicic melt detected by shear wave attenuation at depths of 4–5 km beneath the resurgent dome have probably not been in place long enough to influence sensibly the overlying thermal regime within the upper 2 km of caldera fill.