The majority of vertebrates are not tolerant to hypoxia but epaulette sharks (Hemiscyllium ocellatum) living on shallow reef platforms appear to tolerate hypoxic periods during tidal fluctuations. The effects of progressive hypoxia on the metabolic and ventilatory responses of these elasmobranchs were examined in a closed respirometer. In order to determine whether repeated exposure to hypoxia primes these sharks to alter their metabolism, one group of sharks was exposed to repeated sub-lethal hypoxia, at 5% of air saturation, prior to respirometry. In response to falling oxygen concentration [O2] , the epaulette shark increased its ventilatory rate and maintained its O2 consumption rate (VO2) down to 2.2 mg O/l at 25 degrees C. This is the lowest critical O2 ever measured for any elasmobranch. After reaching the O2crit of 2 22, the shark remained in the respirometer for a further 4.5 h of progressive hypoxia. Only after the O2crit fell to 1.0 mg/l was there a decrease in the ventilatory rate followed by a rise in blood lactate levels, indicating that the epaulette shark responds to severe hypoxia by entering a phase of metabolic and ventilatory depression. Interestingly, hypoxia tolerance was dynamic because hypoxic pre-conditioning lowered the VO2 of the epaulette shark by 29%, which resulted in a significantly reduced O2crit (1.7 mg O/l), revealing that hypoxic pre-conditioning elicits an enhanced physiological response to hypoxia. 2002 Elsevier Science Inc. All rights reserved. Keywords: Hypoxia tolerance; Ventilatory depression; Metabolic depression; Hypoxic pre-conditioning; Respirometry; Elasmobranch; Adaptive physiology