Polyethylene cannulae were implanted in pre- and post-branchial blood vessels allowing nonstressful blood sampling over a variety of activity ranges in an active tropical elasmobranch, the lemon shark (Negaprion brevirostris). TheP 50 was found to be 11.8 Torr at 24°C and pH of 7.7. A Bohr shift of -0.36 was also found. BloodP o 2 and oxygen content were measured during rest, routine swimming, and exercise in unanesthetized, free swimming juveniles. Under all conditions venous oxygen levels were low with venousP o 2 of 7.1±2.7 Torr, and venous oxygen content (  ) of 0.56±0.4 vol%. However, a large variability was found in arterial blood measurements. ArterialP o 2 ranged from 7 to 80 Torr, while arterial oxygen content (Cao 2) varied from 1.6 vol% to 6.8 vol% among ten experimental animals. A significant increase in arterialP o 2, oxygen content, and hematocrit was noted during increased activity. Since the venous system provides little or no oxygen reserve, increased oxygen extraction from the blood (  ) appears to be met by an increase inCao 2 rather than a decrease in  . Mechanisms to accomplish this may include increasing hematocrit and vacular gill shunts.