Rana catesbeiana Shaw tadpoles and adults were maintained at 20-23 degrees C under aerial and aquatic normoxia (PO2 150 mmHg), hyperoxia (PO2 275 mmHg) and hypoxia (PO2 75 mmHg) for 4 weeks, after which the following blood measurements were made: haematocrit, red blood cell count, haemoglobin concentration, mean corpuscular haemoglobin concentration, O2 capacity, O2 equilibrium curve, Bohr shift, Hill's coefficient and intraerythrocytic concentration of nucleotide triphosphates (ATP + GTP) and 2,3-DPG. Normoxic tadpoles had much higher blood O2 affinity (P50 9-10 mmHg) than adults (P50 35 mmHg) but a lower haemoglobin concentration, haematocrit and O2 capacity. The concentration of intraerythrocytic phosphates was higher in normoxic tadpoles than in adults, indicating that the higher O2 affinity of normoxic tadpole blood was due to the haemoglobins themselves, rather than affinity modulators. Chronic hypoxia in tadpoles produced little change in whole blood P50, and no significant change in any other blood variable. In adult bullfrogs, on the other hand, O2 capacity doubled through polycythaemia, and the P50 decreased by 11 mmHg (35%), though apparently not from any significant change in concentration of intraerythrocytic phosphates. Hyperoxia produced no haematological changes in either larvae or adults. In adult bullfrogs exposed to chronic hypoxia, the morphology of the gas exchange organs does not change (Burggren & Mwalukomo, 1983), but instead profound adjustments occur in the blood, favouring O2 transport under these conditions. The blood of the tadpole shows little or no response to chronic hypoxia, with morphological adjustments in skin, gills and lungs constituting the major response.