Although the majority of the sea lamprey's (Petromyzon marinus) life cycle is spent as a burrow-dwelling larva, or ammocoete, surprisingly little is known about intermediary metabolism in this stage of the lamprey's life history. In this study, larval sea lampreys (ammocoetes) were vigorously exercised for 5 min, and their patterns of metabolic fuel depletion and replenishment and oxygen consumption, along with measurements of net whole-body acid and ion movements, were followed during a 4-24-h postexercise recovery period. Exercise led to initial five- to sixfold increases in postexercise oxygen consumption, which remained significantly elevated by 1.5-2.0 times for the next 3 h. Exercise also led to initial 55% drops in whole-body phosphocreatine, which was restored by 0.5 h, but no significant changes in whole-body adenosine triphosphate were observed. Whole-body glycogen concentrations dropped by 70% immediately following exercise and were accompanied by a simultaneous ninefold increase in lactate. Glycogen and lactate were quickly restored to resting levels after 0.5 and 2.0 h, respectively. The presence of an associated metabolic acidosis was supported by very high rates of metabolic acid excretion, which approached 1,000 nmol g(-1) during the first 2 h of postexercise recovery. Exercise-induced ion imbalances were also rapidly alleviated, as initially high rates of net Na(+) and Cl(-) loss (-1,200 nmol g(-1) h(-1) and -1,800 nmol g(-1) h(-1), respectively) were corrected within 1-2 h. Although larval sea lampreys spend most of their time burrowed, they are adept at performing and recovering from vigorous anaerobic exercise. Such attributes could be important when these animals are vigorously swimming or burrowing as they evade predators or forage.