Heart rate, body temperature, arterial blood gas tensions, pHa and hemoglobin concentrations were measured during and after free diving of the Weddell seal (Leptonychotes weddelli), beneath the fast ice of McMurdo Sound, Antarctica. To examine arterial nitrogen tensions (PaN2), 47 samples of arterial blood were obtained from 4 seals diving up to 23 min to depths of 230 m. Peak arterial nitrogen tensions between 2000 and 2500 mmHg were recorded at depths of 40-80 m during descent, indicating that the seal's lung collapses by 25-50 m. Then arterial blood nitrogen tensions slowly decreased to about 1500 mmHg near surfacing. Thus during diving alveolar collapse and redistribution of blood nitrogen allow the seal to avoid nitrogen narcosis and decompression sickness. The arterial PO2 (PaO2) at rest (78 +/- 13 mmHg, mean +/- SD) increased with diving compression to a maximum measured value of 232 mmHg and then rapidly decreased to 25-35 mmHg. The lowest diving PaO2 was 18 mmHg just before the seal surfaced from a 27-min dive. A consistent increase of arterial hemoglobin concentrations occurred during each dive. We suggest that an extension of the sympathetic outflow of the diving reflex caused profound contraction of the Weddell seal's spleen.