Lactate, glycogen, and high-energy phosphate levels were measured in serial biopsies from tuna white muscle during recovery from 15 min of enforced swimming. Exercise caused glycogen and phosphocreatine levels to decrease sharply and lactate concentration to increase markedly (up to 150-mu-mol . g-1). Lactate was cleared from white muscle in less than 90 min, at rates comparable to those seen in mammals (about 1.3-mu-mol . g-1 . min-1), and this was accompanied by nearly stoichiometric increases in white muscle glycogen (2 lactate : 1 glucosyl unit). The plasma lactate concentration remained elevated (35 - 40 mM) until lactate clearance from white muscle was completed, whereas the level of plasma glucose was constant (12 - 16 mM) for the entire 3-h recovery period. The exercise routine caused minimal changes in white muscle purine nucleotides apart from a slight, but significant, increase in IMP content. Transient changes in ATP appear to have resulted from short-term intense swimming activity noted during anesthetization. Unlike other teleosts, lactate clearance in tuna paralleled creatine rephosphorylation during recovery from exercise. We suggest that the postexercise adjustment of intracellular pH is responsible for this relationship. Lactate was seemingly metabolized within the white muscle mass, as indicated by in situ conservation of lactate carbon apparent from stiochiometric increases in white muscle glycogen levels. This prospect is discussed in view of low estimates of lactate utilization rates by other tissues and contrasted with expected high rates of whole-body lactate turnover during recovery.