The metabolic capabilities of the hydrothermal vent tube worm Riftia pachyptila to tolerate short- and long-term exposure to hypoxia were investigated. After incubating specimens under anaerobic conditions the metabolic changes in body fluids and tissues were analyzed over time. The tube worms tolerated anoxic exposure up to 60 h. Prior to hypoxia the dicarboxylic acid, malate, was found in unusually high concentrations in the blood (up to 26 mM) and tissues (up to 5 pmol g(-1) fresh wt). During hypoxia, most of the malate was degraded very quickly, while large quantities of succinate accumulated (blood: about 17 mM; tissues: about 13 mu mol g(-1) fresh wt). Volatile, short-chain fatty acids were apparently not excreted under these conditions. The storage compound, glycogen, was mainly found in the trophosome and appears to be utilized only during extended anaerobiosis. The succinate formed during hypoxia does not account for the use of malate and glycogen, which possibly indicates the presence of yet unidentified metabolic end products. Glutamate concentration in the trophosome decreased markedly during hypoxia, presumably due to a reduction in the autotrophic function of the symbionts during hypoxia. In conclusion, R. pachyptila is physiologically well adapted to the oxygen fluctuations frequently occurring in the vent habitat.