To investigate the biochemical effects of capture and restraint on sharks, 17 serum constituents were measured in three species (bonnethead shark Sphyrna tiburo, blacktip shark Carcharhinus limbatus, and bull shark C. leucas) after gill-net capture. The relative degree of capture effects on each animal was judged using an index of behavioral response devised for use in tag−recapture studies. Serum from each shark was assayed for glucose, creatinine, uric acid, sodium, chloride, potassium, inorganic phosphate, total and ionized calcium, total protein, albumin, globulin, alkaline phosphatase, lactate, lactate dehydrogenase, aspartate aminotransferase, and total iron. In addition, hematocrit was measured from whole blood for each shark. When correlated with the relative degree of capture effects, there were significant intraspecific changes in the concentration of potassium, lactate, inorganic phosphate, uric acid, alkaline phosphatase, aspartate aminotransferase, total and ionized calcium, and glucose. Significant interspecific differences in the concentration of sodium, chloride, potassium, total protein, albumin, globulin, lactate dehydrogenase, aspartate aminotransferase, ionized calcium, alkaline phosphatase, and glucose in minimally stressed animals also were observed. The study suggests that the deleterious effects of gill-net capture and restraint probably involve respiratory and metabolic acidosis and hypoglycemia as well as cellular damage. Species-specific and individual differences in the mortality of sharks caught in gill nets are likely related to an animal's respiratory physiology and degree of struggling upon capture as well as to the extent of net entanglement around the gill area.